feat(sdd-validate): gate PR validation to suite branches

[norrietaylor]

Problem

sdd-validate triggers on pull_request: [opened, synchronize] with no provenance filter, so it posts a findings comment on every PR — dependabot bumps, external contributions, unrelated human PRs. Non-suite PRs resolve to the implementation boundary and get commented on (and can draw needs-human on a protected-path or credential hit).

Fix

Gate the pull_request path to suite branches by head-ref prefix (spec/, arch/, sdd/):

• Job-level if: so non-suite PRs spawn no runner.
• Re-checked in the route script as defense in depth (mirrors the existing parent-issue check on the issues path).

Suite PRs carry no distinguishing label (the lifecycle label lives on the tracking issue), so the head ref is the signal — matching CLAUDE.md branch conventions and the head-ref routing in sdd-execute/sdd-dispatch.

The issues path is already gated to labeled sdd:ready on a parentless tracking issue; not a noise source, unchanged.

Verification

• YAML parses; embedded github-script passes node --check
• startsWith(github.event.pull_request.head.ref, …) matches the expression form already used in sdd-dispatch.yml/sdd-execute-*.yml

🤖 Generated with Claude Code

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 6a58c1a9-f8a0-410d-bef0-4423800a0a17

📥 Commits

Reviewing files that changed from the base of the PR and between cf15161 and f13280d.

📒 Files selected for processing (1)

• wrappers/sdd-validate.yml

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/sdd-validate-branch-gate

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