v0.15.1: ARCH-1 step 1 — extract RcloneClient collaborator

[NathanNeurotic]

Summary

First incremental step toward the audit's ARCH-1 ("`TrayContext` is a ~6 k-line god object"). Pulls all rclone process invocation behind a single `RcloneClient` class so future code can take it as a dependency without dragging in the rest of `TrayContext`.

No user-visible behavior changes — public API unchanged, every existing caller still works.

Changed

• New `src/RcloneClient.cs` owns the "invoke rclone, return `ProcessResult`" surface:
  • `Run(arguments, timeoutMs)` and `Run(arguments, timeoutMs, envOverrides)` — replaces the inline `new ProcessStartInfo { ... }` boilerplate
  • `RunWithStdin(arguments, timeoutMs, stdin)` — for SEC-1's `rclone obscure -` pipe
  • Constructor takes a `Func` path provider so the client always sees the current `rclonePath` (which mutates over the app's lifetime as the user downloads / installs rclone)
• `TrayContext.RunRcloneCapture` wrappers delegate to a lazy `RcloneInvoker` field.* Public API unchanged; existing callers don't need to change. New code should take `RcloneClient` directly for testability.
• SEC-1 stdin path (`ObscureSecretViaStdin`) uses `RcloneInvoker.RunWithStdin` — no longer reaches into `RunCaptureCore` directly.

The remaining ARCH-1 sub-extractions (`SettingsStore`, `DependencyProbe`, `MountManager`) are intentionally not done here. The audit calls this work "incremental"; the rest can land in later releases as the natural shape of each collaborator emerges.

This closes the last finding from the audit — 17 of 17 audit items resolved or explicitly deferred-by-incremental-design.

Test plan

• `scripts/run-tests.ps1` — 53/53 green
• FileVersion 0.15.1.0
• Real-world: install v0.15.1, do anything that touches rclone (mount, refresh, configure a new remote) — should work identically to v0.15.0

🤖 Generated with Claude Code

[gemini-code-assist]

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