Propagate $GITHUB_PATH into chroot PATH for setup-* action support

[Copilot]

Tools installed by pre-AWF setup steps (setup-ruby, setup-dart, etc.) aren't visible inside the chroot because $GITHUB_PATH entries may be lost when sudo resets PATH, and the fallback PATH only hardcoded Node/Python/Go from hostedtoolcache.

Note: /opt/hostedtoolcache is already bind-mounted (/opt:/host/opt:ro) — the binaries exist on disk, they're just not in PATH.

src/docker-manager.ts

• readGitHubPathEntries() — reads the $GITHUB_PATH file written by setup-* actions
• mergeGitHubPathEntries() — prepends those entries into PATH with deduplication
• AWF_HOST_PATH now merges $GITHUB_PATH file contents with process.env.PATH

if (process.env.PATH) {
  const githubPathEntries = readGitHubPathEntries();
  environment.AWF_HOST_PATH = mergeGitHubPathEntries(process.env.PATH, githubPathEntries);
}

containers/agent/entrypoint.sh

• Fallback PATH (when AWF_HOST_PATH is unset) now dynamically scans all /opt/hostedtoolcache/*/ tool directories instead of hardcoding only three tools

Tests

• 14 new unit tests covering readGitHubPathEntries, mergeGitHubPathEntries, and generateDockerCompose integration with $GITHUB_PATH

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