[codex] Add project advisory PR comments

[Vilin97]

Summary

Adds an advisory PR comment workflow that posts source-project metadata for each open PR, including drafts. The comment reports the imported project's GitHub-reported license, default-branch last commit date, and source repo lean-toolchain version, then updates the same sticky comment on reruns.

Details

• Adds lean_pool.pr_advisory, a deterministic GitHub API-based commenter that never checks out or runs PR-head code.
• Adds a pull_request_target, manual, and scheduled workflow to keep advisory comments current.
• Adds tests for project-entry detection, source metadata fetching, draft-inclusive PR listing, rendering, and sticky comment updates.

Validation

• cd python && uv run ruff format --check && uv run ruff check && uv run --group test pytest
• actionlint
• Live dry run across open PRs before posting
• Posted and verified one advisory marker comment on every open PR, including draft PR [import] Kaprekar4 (WIP) #223

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