feat: adopt .agent.md naming convention for Copilot backend

[jafreck]

Summary

• feat: adopt .agent.md naming convention for the Copilot backend
• fix: add max bounds to maxParallelIssues (20) and maxParallelAgents (10) in config schema
• ci: add canonical labels.yml and label-sync workflow
• chore: lower maxParallelIssues to 10 and maxParallelAgents to 4 in cadre.config.json

All 1768 tests pass.

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 65.21739% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.18%. Comparing base (930f73a) to head (503bd8e).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/core/agent-launcher.ts	50.00%	4 Missing ⚠️
src/cli/agents.ts	83.33%	2 Missing ⚠️
src/index.ts	0.00%	2 Missing ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##             main      #95      +/-   ##
==========================================
- Coverage   76.26%   76.18%   -0.08%     
==========================================
  Files          70       70              
  Lines        6859     6875      +16     
  Branches      894      899       +5     
==========================================
+ Hits         5231     5238       +7     
- Misses       1610     1619       +9     
  Partials       18       18              

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[jafreck]

Code Review — REQUEST CHANGES

The .agent.md naming convention feature itself is correctly implemented and consistently applied across cli/agents.ts, agent-launcher.ts, index.ts, and the test updates. However, the PR bundles unrelated changes that must be separated before merge.

🔴 Blockers

1. Schema guardrail removal is a silent safety regression (unrelated to this PR)
src/config/schema.ts removes .max(100) on limit, .max(20) on maxParallelIssues, and .max(10) on maxParallelAgents. With the config bumped to maxParallelIssues: 100 / maxParallelAgents: 10, this opens the door to 1,000 simultaneous agent processes with no schema validation backstop. These changes need their own PR with an explicit justification.

2. Config escalation bundled into a naming-convention PR
cadre.config.json bumps limit 20 → 1000 and maxParallelIssues 8 → 100. These are 12.5× and 5× operational increases with no rationale in this PR. Separate PR, please.

🟡 Medium Issues

3. Stale test description in tests/cli-agents.test.ts
The test description still says <agentDir>/<name>.md but the assertion correctly checks .agent.md. Must be updated.

4. Missing test coverage for validateAgentFiles() with backend = 'claude' and fallback
The three-branch logic in agent-launcher.ts has only one branch tested (default 'copilot'). Add tests for backend = 'claude' (should look for <name>/CLAUDE.md) and explicit backend = 'other' (fallback .md).

🔵 Minor

5. DRY: config.agent?.backend ?? 'copilot' is inlined in index.ts rather than calling resolveBackend(). Since resolveBackend is already defined, wire it through or export it.

6. No migration note for users who scaffolded .md files before this change (the error message is helpful but a CHANGELOG entry would be better UX).

What's Correct

• The three-way naming logic is identical between agentFilePath() and validateAgentFiles() — no drift.
• The resolveBackend() helper correctly handles the agent block being entirely absent from config.
• All primary tests are updated to match the new default.