[#805] Tighten trigger message — RE1/RE2 review only @mentioned PRs

[realproject7]

Part of #805 (step 3).

Two separable changes — Part A is independent and can land first; Part B depends on #807.

Part A — tighten reviewer scope (INDEPENDENT, no dependency)

The trigger prompt tells RE1/RE2 to "Review open PRs," which drives blanket gh pr list discovery every cycle. In DEFAULT_MESSAGE / per-project trigger_message (server/index.js), change RE1/RE2 to: "review only PRs you were @mentioned on." This cuts reviewer discovery immediately, with no dependency on GITHUB.md.

Part B — point discovery at GITHUB.md (DEPENDS ON #807)

Change all roles to discover issue/PR state from GITHUB.md / GET /api/github-parsed instead of gh pr list. This must not ship before #807 (the file/endpoint must exist). If shipped during rollout, include fallback wording: "if GITHUB.md is absent, fall back to a single gh discovery call."

Risk: Low.

Acceptance criteria

• Part A: trigger message no longer instructs blanket "review open PRs"; RE1/RE2 act only on @mentioned PRs (can land independently)
• Part B: discovery points at GITHUB.md / /api/github-parsed, gated on [#805] GITHUB.md template + seed + endpoints + parser + test #807 (or carries the absent-file fallback)
• npm run build passes

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Workflow-risk guardrails (from #805 risk analysis)

• Reword the trigger message to point at GITHUB.md / /api/github-parsed, no-op quietly on empty, and carry a staleness caveat ("GITHUB.md may be stale — confirm with a direct gh read before merge/review").
• Verify the per-cycle trigger enumerates the PR number + author so "review only @mentioned PRs" has a concrete target to bind to.
• Extend the Part B fallback from "absent file" to "absent OR stale > 2 cycles" → fall back to one direct gh read.