feat(packaging): Linux AppImage installer

[rrader26]

Summary

• Linux `.AppImage` installer scaffolding — completes the cross-platform matrix (macOS .pkg, Windows .msi, Linux .AppImage).
• GitHub Actions `build-linux` job in the release workflow; one tag push produces all three artifacts.
• Launcher script updated to handle BOTH the macOS pkg layout AND the Linux AppImage layout (probes for the entry file).

What ships

Each `.AppImage` is one self-contained file. User makes it executable, double-clicks (or runs from a terminal), they're on a working `agentmark-mcp`.

```
AgentMark.AppDir/
├── AppRun entrypoint (delegates to launcher)
├── agentmark.desktop XDG desktop entry (AppImage spec)
├── agentmark.png placeholder icon (AppImage spec)
└── usr/
├── bin/
│ ├── node pinned Node 22.11.0 LTS
│ └── agentmark-mcp launcher script
├── lib/agentmark/ dist/ + schema/ + package.json + prod deps
└── share/
├── applications/agentmark.desktop
└── icons/hicolor/256x256/apps/agentmark.png
```

What's NOT in the Linux build

• Desktop a11y bridge. AT-SPI Linux bridge isn't in this codebase yet. Every other plugin (memory, recipes, network, foundations, vision, system, microsoft, web, pdf) runs fine.
• When the AT-SPI bridge ships, it slots into the same AppDir layout at `usr/lib/agentmark/bridges/agentmark-bridge-linux`. The launcher already detects + wires it.

Launcher dual-layout support

The shared `launcher.sh` now handles both:

Layout	Entry resolution
macOS pkg	`/bin/agentmark-mcp` → `/node` + `/agentmark/dist/...`
Linux AppImage	`/usr/bin/agentmark-mcp` → `/usr/bin/node` + `/usr/lib/agentmark/dist/...`

Also: launcher now only sets `AGENTMARK_BRIDGE_PATH` when the bridge binary actually exists. Previously hardcoded the macOS bridge name — on Linux that env var would point at a missing file.

Test plan

• Shell scripts syntax-validated (`bash -n`)
• Both launcher layouts logic-traced
• Manual: trigger the release workflow on a tag and verify all three artifacts upload to a draft Release
• Manual: download the .AppImage, `chmod +x`, run `./AgentMark-x.y.z-linux-x86_64.AppImage --help` and `./AgentMark... install --dry-run` on a fresh Ubuntu VM

Pinned versions (unchanged)

Node	22.11.0 LTS
Tooling	`appimagetool-x86_64.AppImage` continuous build (auto-downloaded by the script)

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