feat(cli): add self-update command for automatic binary updates

[gcmartinezg]

📋 Pre-flight Checks

• I have searched existing issues and this is not a duplicate
• I understand this issue needs status:approved before a PR can be opened

🔍 Problem Description

I recently encountered a critical issue where my local engram installation was running an old version (pre-v1.10) with the 2000-character truncation limit, while the current codebase has a 50,000-character limit (fixed in commit 4235fa6).

This resulted in:

• Data loss: Most my observations were permanently truncated at exactly 2000 characters
• Silent degradation: The version checker notified me of updates, but I had to manually rebuild from source
• Inconsistent behavior: Different installations could have vastly different limits without users realizing

The current update workflow requires users to either:

1. Run go install github.com/Gentleman-Programming/engram/cmd/engram@latest (requires Go toolchain)
2. Manually download from GitHub releases (requires finding the correct asset for their platform)
3. Use Homebrew (macOS/Linux only, not everyone uses it)

This creates friction and increases the risk of running outdated versions with bugs or missing features.

💡 Proposed Solution

Add a self-update command that automatically downloads and installs the latest engram binary:

engram update

Expected behavior:

1. Fetch latest release from GitHub API (already implemented in internal/version/check.go)
2. Download the appropriate binary for the current platform (Windows/macOS/Linux, amd64/arm64)
3. Replace the running binary with the new version
4. Verify the update succeeded
5. Display the new version and changelog

Example output:

Checking for updates...
Found new version: v1.10.1 (current: v1.9.5)

Downloading engram_1.10.1_windows_amd64.zip...
[████████████████████████████████████] 100%

Installing to C:\Users\user\bin\engram.exe...
✓ Update successful!

engram v1.10.1
Release notes: https://github.com/Gentleman-Programming/engram/releases/tag/v1.10.1

Optional flags:

• --check: Only check for updates, don't install
• --force: Force update even if already on latest version
• --version <tag>: Install a specific version

Technical approach:

• Use GitHub Releases API to find latest version
• Download the appropriate asset based on runtime.GOOS and runtime.GOARCH
• Use a library like go-selfupdate or equinox.io for safe binary replacement
• Implement atomic replacement to avoid corrupting the binary on failure

📦 Affected Area

CLI (commands, flags)

🔄 Alternatives Considered

Alternative 1: Keep manual update only

• ❌ Requires users to remember to update
• ❌ Risk of running outdated versions with critical bugs
• ❌ Friction for non-technical users

Alternative 2: Auto-update on every run

• ❌ Too aggressive, could surprise users
• ❌ Might fail in restricted environments
• ✅ Current version checker is a good middle ground (notify but don't install)

Alternative 3: Package manager only (Homebrew, apt, etc.)

• ❌ Not all users use package managers
• ❌ Windows users have limited options
• ❌ Doesn't help users who installed via go install

📎 Additional Context

Real-world impact:

• My observations on a few of the projects I have been working on were truncated at 2000 chars and cannot be recovered
• The version checker alerted me to updates, but I still had to figure out how to update manually
• A simple engram update command would have prevented this data loss

Reference implementations:

• Goreleaser has built-in self-update
• gh CLI has gh upgrade
• hugo has similar functionality via plugins

Code locations:

• Version checking already exists: internal/version/check.go
• Just needs download + install logic added