ci: replace auto-triggered native builds with comment-triggered slash commands

[Copilot]

Native builds were running on every PR open/synchronize, consuming expensive macOS/Windows runners unconditionally. They should only run when explicitly requested.

Changes

• Removed pull_request trigger from pr-build-native.yml — native builds no longer fire automatically on PR events
• Added issue_comment trigger — workflow activates when a PR comment contains one of:
  • /build-native-linux → runs ubuntu-latest
  • /build-native-mac → runs macos-latest
  • /build-native-windows → runs windows-latest
• Split matrix into 3 independent jobs (build-linux, build-mac, build-windows), each gated by github.event.issue.pull_request != null + comment body check — individual platforms can be triggered independently
• Explicit PR head SHA checkout — fetches the PR's head.sha via the GitHub API before checkout, since issue_comment events carry no PR ref
• Scoped permissions — added permissions: contents: read to all build jobs; comment and notify-start jobs use issues: write
• Build status comments — a notify-start job immediately posts a "🚀 Native build starting" comment on the PR listing the triggered platforms and a link to the running job; the comment job then updates that same comment with per-platform ✅/❌ results, an artifact download link on success, or a log link on failure

[sunix]

/build-native-linux

[sunix]

@copilot should add a comment saying that it is starting with the job url, and update the same comment once it is done with the url link of the downloadable url, or saying that there is an error with the job url.

[github-actions]

📦 Native binaries available

The native binaries for this PR have been built and are available for download:

➡️ Download artifacts from this Actions run

Artifacts expire after 1 day.

[Copilot]

@copilot should add a comment saying that it is starting with the job url, and update the same comment once it is done with the url link of the downloadable url, or saying that there is an error with ...

Done in commit 0f0cfbb. Here's how it works now:

1. On trigger — a new notify-start job immediately posts a comment like:

   🚀 Native build starting
   Building native binaries for: Linux
   ⏳ Build in progress... View job →

2. On completion — the comment job updates that same comment with per-platform results:

   🏁 Native build results
   ✅ Linux: success
   ➡️ Download artifacts from this Actions run — Artifacts expire after 1 day.

   Or on failure:

   🏁 Native build results
   ❌ Linux: failure
   🔍 View job logs

[sunix]

/build-native-linux