feat(code-audit): add PR auditor as second code-audit slice

[W00DSRULES]

openworkers audit pr <github-url> extracts atomic claims from a PR description and verdicts each against the actual unified diff. Same pipeline shape as the README auditor (planner → deterministic researcher → checker + trust gate → critic), proving that the SourceAdapter abstraction generalises to a second backend.

New modules:

• core/sources/github.py — GitHubAdapter over a unified diff via a PrSpec value object; live fetch (fetch_pr_from_github) is a sibling helper, decoupled from the adapter so tests stay network-free. parse_pr_url + load_pr_fixture as supporting helpers.
• core/orchestrator/pr_flow.py — PrAuditOrchestrator.
• core/orchestrator/audit_prompts.py — shared audit-prompt loader (extracted from readme_flow.py so each new auditor registers its templates in one place).
• prompts/code_audit/pr_planner.md + pr_checker.md — PR-specific claim types (add | remove | fix | refactor | test | behavior | doc | other) and diff-aware verdict rules.

Schema:

• core/schemas_audit.py exposes AuditClaim / AuditClaimList as aliases of ReadmeClaim / ReadmeClaimList so PR code reads cleanly without churning the README slice. AuditReport.target captures the audited artefact id (PR URL, etc.).

Agents:

• providers/code_audit_agents.py adds PrPlannerAgent + PrCheckerAgent alongside the README versions. The same _enforce_trust_gate runs after the LLM responds, so any claim with no diff evidence is forced to unsupported regardless of LLM output. AuditCriticAgent is reused as-is.

CLI:

• openworkers audit pr <url> (or --fixture <dir> for offline runs) with optional --token falling back to GITHUB_TOKEN / GH_TOKEN.

Tests:

• tests/fixtures/sample_pr/ — canned PR JSON + unified diff with one verified, one drifted, one contradicted, one fabricated (no-evidence) claim.
• tests/code_audit/test_pr_flow.py — URL parser, fixture loader, adapter grep, end-to-end audit with verdict distribution, and an explicit trust-gate-override assertion for the hallucinated WIDGETLIB_DEBUG claim.

Docs: README.md, ROADMAP.md, CHANGELOG.md, AGENTS.md updated with the new slice, shared pipeline section, and PR-audit-flow walkthrough.

Verification: 159/159 tests pass (153 existing + 6 new PR tests), mypy strict clean on new modules, ruff clean on new files, black formatted. CLI smoke-runs in text and JSON modes against the fixture.

Summary

Changes

Testing

• pytest tests/ -v passes
• ruff check . passes
• black --check . passes
• mypy core/ providers/ --strict --ignore-missing-imports passes

Checklist

• I have read CONTRIBUTING.md
• Code follows project style (black, ruff, mypy strict for core/providers)
• Tests added or updated for the changes
• Documentation updated if needed