docs: add AGENTS contribution guide

AGENTS.md

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+You are an experienced, pragmatic software engineering AI agent. Do not over-engineer a solution when a simple one is possible. Keep edits minimal. If you want an exception to ANY rule, you MUST stop and get permission first.
+
+# Project Overview
+
+This repository contains the Linux-focused Coder Desktop implementation in C#/.NET. The app provides tray-based access to Coder workspaces through a local VPN-like tunnel (`Coder Connect`) and includes Linux packaging assets for distribution.
+
+Primary goals:
+- Provide a Linux desktop/tray UX for connecting to Coder workspaces.
+- Coordinate with a local/root VPN service over RPC.
+- Ship installable Linux artifacts (`.deb`, `.rpm`, `.tar.gz`, and AUR source bundle).
+
+Technology choices:
+- **Language/runtime:** C# on .NET 8 (`net8.0` across projects)
+- **UI:** Avalonia 11 (`App.Avalonia`)
+- **Architecture:** DI with `Microsoft.Extensions.*`, shared ViewModel/service layer in `App.Shared`
+- **RPC/protocol:** Protobuf messages (`Vpn.Proto/vpn.proto`) + custom stream RPC (`Vpn/Speaker.cs`)
+- **Logging:** Serilog + Microsoft.Extensions.Logging
+- **Testing:** NUnit (`Tests.*` projects)
+- **Packaging:** Bash scripts under `Packaging.Linux/`
+
+# Reference
+
+## Important files
+
+- `Coder.Desktop.sln` — solution entrypoint for all projects
+- `App.Avalonia/App.axaml.cs` — application composition root (DI/service registration, startup flow)
+- `App.Avalonia/Program.cs` — UI process entrypoint + bootstrap logging
+- `App.Shared/Services/RpcController.cs` — app-side RPC lifecycle/state orchestration
+- `Vpn/Speaker.cs` — generic request/reply protocol transport over streams
+- `Vpn.Proto/vpn.proto` — protocol contract used by app/service/tunnel
+- `Vpn.Service/Program.cs` — VPN service host, config sources, platform transport wiring
+- `scripts/run-linux-dev.sh` — local Linux dev launcher (service + app)
+- `.github/workflows/ci.yaml` — CI source of truth for baseline build/test checks
+- `.editorconfig` — formatting/style baseline (including line endings)
+
+## Important directories
+
+- `App.Avalonia/` — Avalonia UI, views, controls, Linux desktop host wiring
+- `App.Shared/` — cross-UI application logic (ViewModels/Models/Services abstractions)
+- `App.Linux/` — Linux-only service implementations (startup, credential backend, notifications)
+- `Vpn.Service/` — long-running manager service (systemd-compatible on Linux)
+- `Vpn/`, `Vpn.Linux/`, `Vpn.Proto/` — RPC core, Linux transports, protobuf schema
+- `CoderSdk/`, `MutagenSdk/` — API clients and vendored proto-related integration code
+- `Tests.CoderSdk/`, `Tests.Vpn*/` — NUnit unit/integration-style test projects
+- `Packaging.Linux/` — package build scripts, unit files, desktop metadata
+- `scripts/` — developer automation scripts
+
+## Project architecture (high level)
+
+1. `App.Avalonia` boots and builds DI container in `App.axaml.cs`.
+2. `RpcController` in `App.Shared` connects to the local service via `IRpcClientTransport` (`UnixSocketClientTransport` on Linux).
+3. RPC is executed via `Speaker<TSend,TReceive>` request/reply framing and protobuf payloads.
+4. `Vpn.Service` hosts service logic (`Manager`, `TunnelSupervisor`, etc.) and exposes RPC over Unix socket.
+5. Packaging scripts stage/publish both app and service binaries into Linux package formats.
+
+# Essential commands
+
+Run all commands from repository root.
+
+## Build
+
+```bash
+dotnet restore Coder.Desktop.sln
+dotnet build Coder.Desktop.sln -c Release --no-restore
+```
+
+## Format
+
+```bash
+dotnet format Coder.Desktop.sln
+```
+
+## Lint
+
+No dedicated linter script exists. Use formatting/analyzer verification:
+
+```bash
+dotnet format Coder.Desktop.sln --verify-no-changes
+```
+
+## Test
+
+Match CI coverage:
+
+```bash
+dotnet test Tests.CoderSdk/Tests.CoderSdk.csproj --no-restore -v q
+dotnet test Tests.Vpn.Proto/Tests.Vpn.Proto.csproj --no-restore -v q
+dotnet test Tests.Vpn/Tests.Vpn.csproj --no-restore -v q
+dotnet test Tests.Vpn.Service/Tests.Vpn.Service.csproj --no-restore -v q
+```
+
+## Clean
+
+```bash
+dotnet clean Coder.Desktop.sln
+```
+
+## Development server / local run
+
+```bash
+./scripts/run-linux-dev.sh --show --sudo-service
+```
+
+Useful variants:
+
+```bash
+./scripts/run-linux-dev.sh --no-build -- --minimized
+./scripts/run-linux-dev.sh --help
+```
+
+## Other important scripts
+
+```bash
+# Multi-format Linux packages (deb/rpm/tar)
+VERSION=1.2.3 ./Packaging.Linux/build-release-packages.sh amd64
+
+# AUR source bundle
+VERSION=1.2.3 ./Packaging.Linux/build-aur-source.sh
+
+# Deb-only helper wrapper
+VERSION=1.2.3 ./Packaging.Linux/build-deb.sh amd64
+
+# Refresh vendored Mutagen proto tree
+pwsh ./MutagenSdk/Update-Proto.ps1 -mutagenTag <tag>
+```
+
+# Patterns
+
+- **Keep App.Shared UI-framework-agnostic.**
+  - Do: expose abstractions (`IWindowService`, `IDispatcher`) and neutral types.
+  - Don’t: instantiate Avalonia-specific UI types in `App.Shared`.
+
+- **UI thread handoff in ViewModels is explicit.**
+  - Do: check `_dispatcher.CheckAccess()` and repost with `_dispatcher.Post(...)` when required.
+  - Don’t: mutate UI-bound observable state from background threads.
+
+- **Avoid redundant property writes in ViewModels.**
+  - Do: compare old/new before assigning to reduce flashing and unnecessary notifications.
+  - Don’t: blindly reassign observable properties during model refreshes.
+
+- **RPC request/reply discipline.**
+  - Do: use `SendRequestAwaitReply` for command flows needing responses and preserve message type checks.
+  - Don’t: ignore unexpected message types or skip lifecycle transitions on failures.
+
+- **Testing style.**
+  - NUnit is standard; async tests commonly include `[CancelAfter(30_000)]` to avoid hangs.
+
+# Anti-patterns
+
+- **Do not wire both tray icon `Command` and click event behavior** in ways that double-trigger UI toggles (`App.Avalonia/App.axaml.cs` explicitly avoids this).
+- **Do not assume unvalidated URLs are safe for UI hyperlink controls**; invalid values can crash UI paths (see `TrayWindowViewModel` comments).
+- **Do not manually edit vendored/generated proto imports in `MutagenSdk/Proto/`** when they originate from Mutagen; regenerate via `Update-Proto.ps1`.
+- **Do not bypass serialization/lock patterns in RPC state management** (`RpcController` uses explicit operation/state locks for consistency).
+
+# Code style
+
+Source of truth: `.editorconfig`
+
+Key rules currently in effect:
+- Default indentation: 4 spaces
+- JSON/YAML indentation: 2 spaces
+- `.proto` indentation: tabs (`indent_size = 1`)
+- Default line endings: `CRLF`
+- UTF-8, trim trailing whitespace, final newline
+- C# style prefers `var` in common scenarios and nullable reference types are enabled
+
+When editing files, match surrounding style and existing naming/structure patterns.
+
+# Commit and Pull Request Guidelines
+
+## Before committing
+
+- Run at minimum the CI-equivalent build/test flow for impacted areas.
+- If you touched shared/protocol/service code, run all test commands listed above.
+- If you touched packaging, run the relevant packaging script(s) and capture outputs/artifact names.
+
+## Commit messages
+
+Current history is minimal and imperative (e.g., `add linux release packaging workflow`).
+
+Preferred format for new commits:
+- `type: concise imperative summary`
+- Examples: `fix: handle rpc reconnect timeout`, `build: adjust linux package dependencies`
+
+If no clear type applies, use a concise imperative summary rather than vague text.
+
+## Pull request description requirements
+
+No PR template is present in this repository, so include:
+- **What changed** (bullet list)
+- **Why** (user/problem impact)
+- **How it was validated** (exact commands + results)
+- **Risk/rollback notes** for service, protocol, or packaging changes
+- **Screenshots/video** for tray/UI behavior changes