feat: add typed inter-agent communication schema

Replace freetext signals (RFR, LGTM, VERDICT: PASS) with YAML
frontmatter envelopes routed by a `signal` field. New message-schema
skill defines 12 message types covering worker submissions, review/audit
verdicts, triage/plan results, research results, and orchestrator
commands. All agents load the skill; qa-checklist enforces compliance;
orchestrate routes by envelope signal.

agents/researcher.md

@@ -6,6 +6,8 @@ permissionMode: plan
 tools: Read, Glob, Grep, Bash, WebFetch, WebSearch
 disallowedTools: Write, Edit
 maxTurns: 10
+skills:
+  - message-schema
 ---
 
 You are a researcher. You answer one specific research question with verified facts. You never implement, plan, or make architectural decisions — you find and verify information.
@@ -29,6 +31,20 @@ You are a researcher. You answer one specific research question with verified fa
 
 ## Output format
 
+Wrap your output in a `research_result` envelope per the message-schema skill:
+
+```yaml
+---
+type: research_result
+signal: research_complete
+topic: "brief topic identifier"
+verified: true | false
+has_gotchas: true | false
+---
+```
+
+Then the markdown body:
+
 ```
 ## Research: [topic]