docs: create Linux support documentation (docs/linux-support.md)

## Summary

There is thorough `docs/windows-support.md` documenting Linux install instructions, test inventory, and known gaps. No equivalent exists for Linux, which is barely mentioned except as a data-root path in workspace tests.

## Current State

- **`docs/windows-support.md`**: Comprehensive Windows support documentation
- **Linux**: Only mentioned in test data as a path example
- **README.md**: States Linux is "not yet supported"

## Missing Documentation

1. Linux support status and roadmap
2. Installation instructions (AppImage, deb, rpm)
3. Known gaps (computer-use, eval harness signals, file permissions)
4. Test inventory for Linux
5. Development setup on Linux
6. Platform-specific considerations (X11 vs Wayland, accessibility)

## Proposed Solution

Create `docs/linux-support.md` mirroring the structure of `docs/windows-support.md`:
- Support status and limitations
- Installation methods
- Known gaps and workarounds
- Test inventory
- Development setup

## Acceptance Criteria

- [ ] Documentation accurately reflects current Linux support status
- [ ] Installation instructions are tested
- [ ] Known gaps are documented with issue references

## Labels

enhancement, documentation

Start by reading docs/windows-support.md and the Linux support statement in README.md to mirror the existing structure and identify the current status. Create docs/linux-support.md covering installation methods, known gaps, test inventory, development setup, and platform considerations. Done means the support status is accurate, installation instructions are tested, and known gaps include issue references.

Written by the indexing model from the issue text.