Add Linux E2E tests and improve release notes (#239)

## Summary

This PR adds comprehensive end-to-end testing for Linux environments and
enhances release documentation by including installation instructions on
GitHub release pages.

## Key Changes

- **New Linux E2E workflow** (`e2e-linux.yml`):
  - Tests on Node.js 24 with parallel execution (6 workers)
  - Tests on Bun runtime with parallel execution (4 workers)
  - Runs without gnome-keyring to test encrypted file storage fallback
- Integrated into the release workflow to ensure Linux compatibility
before publishing

- **Enhanced release workflow**:
  - Added `e2e-linux` job as a required dependency for releases
  - Updated release condition to require successful Linux E2E execution
- Added installation instructions to both pre-release and stable release
GitHub release pages
  - Instructions include both npm and Bun installation methods
  - Output package name to job outputs for reuse in release body

- **Improved release documentation**:
- Install instructions now appear on the release page itself (via
`append_body`)
- Instructions printed to both job logs and run summary for visibility
during execution
- Used `tee` to output to both stdout and GitHub summary simultaneously

## Implementation Details

- Linux E2E tests run in parallel to reduce CI time while maintaining
coverage
- Pre-release and stable release flows both include installation
instructions
- Package name is captured as a job output to avoid hardcoding in
release body templates
- Release condition uses `always()` with explicit result checks to
handle optional Windows E2E while requiring Linux E2E

https://claude.ai/code/session_01KHm3opxBjDudVG9jr5sSTq

Co-authored-by: Claude <noreply@anthropic.com>