feat(review): run on non-PR issue targets (#142 slice 3, ADR-0033)

[stephane-segning]

1. Summary

Completes #142 (ADR-0033): an @mention on a plain issue (not just a PR thread) now launches a task. handle_issue_comment branches on whether the issue carries a pull_request object:

• PR thread → fetch base/head SHAs, target_type=pull_request, diff-scoped review (unchanged).
• Plain issue → target_type=issue, no SHAs.

The rest of the chain already supports it: no diff ⇒ the agent omits add_review_comment and answers via add_comment ⇒ finalize posts a single issue comment (emergent kind ask). The idempotency key includes target_type, so an issue and a PR with the same number are distinct tasks.

Source of truth: #142 / ADR-0033.

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2. Intent

ADR-0033 said run kinds carry a target beyond PRs. The engine already answers diff-less runs (since #140's no-diff tool gating); this just lets the webhook point the agent at an issue, so you can @mention it on a bug report or question and get a grounded reply.

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3. Scope

In Scope

• Webhook accepts issue_comment on a non-PR issue → issue task (no SHAs).

Out of Scope

• Commit/repo targets (ADR-0033 mentions them; not requested).
• Runner changes — none needed (no-diff path already handles it).

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4. Verification

• Running automated tests

DATABASE_URL=postgres://lightbridge:lightbridge@localhost:5432/lightbridge cargo test -p control-plane --locked
cargo fmt --check && cargo clippy -p control-plane --all-targets

control-plane: 57 passed (1 new issue-target round-trip test); webhook unit tests green; fmt + clippy clean

New test: an issue task (no SHAs) round-trips via get_task_context, and a PR with the same number is a distinct task (target_type discriminates the idempotency key).

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5. Screenshots / Evidence

N/A (backend). pr_diff returns None without SHAs (verified) → the diff-less agent path + issue-comment finalize sink (both already in main) carry the rest.

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6. Risk Assessment

• Low — additive webhook branch; PR path unchanged; approval gate still applies to issues.

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7. AI Usage Declaration

AI was used for:

• Understanding existing code
• Generating code
• Generating tests

Human verification:

• I understand every meaningful change in this PR
• I traced the diff-less → add_comment → finalize chain for issues
• I accept responsibility for this PR

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8. Reviewer Focus

• Correctness (PR vs issue branch; approval gate; idempotency key)
• Edge cases (no SHAs, same number PR vs issue)

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[github-actions]

✅ AI Governance check passed

This PR declares AI usage, references a source of truth, and provides verification evidence. Thank you.

[gemini-code-assist]

Code Review

This pull request implements support for handling issue comments on plain GitHub issues (ADR-0033 slice 3) in addition to pull requests. It updates the webhook handler to distinguish between PRs and plain issues, conditionally fetching SHAs only for PRs, and updates the database task creation and logging to use the correct target type. Additionally, a new test has been added to verify that issue tasks round-trip correctly and remain distinct from PR tasks with the same ID. There are no review comments, and I have no feedback to provide.

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