feat(enrichment): forward PR body and linked-issue envelope to REES

[kiannidev]

Problem

The REES history analyzer already accepts body and linkedIssue on /v1/enrich so it can:

• parse Fixes #123 references from the PR body when no token is available
• measure diff coverage against the linked issue's stated requirement without an extra GitHub fetch

The engine's buildReviewEnrichment path never forwards those fields today — only author, files, and diff reach REES.

Proposal

• Extend buildReviewEnrichment to POST body when present
• Resolve the PR's primary linked issue from the engine cache (getIssue) and send a compact linkedIssue envelope { number, title?, body? }
• Wire runAiReviewForAdvisory to pass pr.body and pr.linkedIssues

Acceptance criteria

• REES POST includes body when the PR has one
• REES POST includes linkedIssue when the PR links an issue the engine has cached
• Fail-safe: missing cache/issue data omits linkedIssue (history analyzer degrades gracefully)
• Unit + wiring tests cover the processor → enrichment-wire path

Notes

Parent capability: #1478 / #1697. This is the remaining engine-side passthrough after the REES history analyzer landed on main.