test: Codex generation tests + CI + docs for multi-agent support

Tests (28 new):
- Codex output path routing, frontmatter validation (name+description only)
- No .claude/skills/ path leaks in Codex output (regression guard)
- /codex skill exclusion, hook→prose conversion, multiline YAML
- --host agents alias, dynamic template discovery
- Codex skill validation + $B command validation
- find-browse priority chain verification
- Replace static ALL_SKILLS list with dynamic filesystem scan

CI:
- Add Codex freshness check to skill-docs workflow

Docs:
- AGENTS.md: Codex-facing project instructions
- README: multi-agent installation section
- CONTRIBUTING: dual-host development workflow
- CHANGELOG: v0.9.0 multi-agent support entry

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

browse/test/find-browse.test.ts

@@ -21,4 +21,30 @@ describe('locateBinary', () => {
       expect(existsSync(result)).toBe(true);
     }
   });
+
+  test('priority chain checks .codex, .agents, .claude markers', () => {
+    // Verify the source code implements the correct priority order.
+    // We read the function source to confirm the markers array order.
+    const src = require('fs').readFileSync(require('path').join(__dirname, '../src/find-browse.ts'), 'utf-8');
+    // The markers array should list .codex first, then .agents, then .claude
+    const markersMatch = src.match(/const markers = \[([^\]]+)\]/);
+    expect(markersMatch).not.toBeNull();
+    const markers = markersMatch![1];
+    const codexIdx = markers.indexOf('.codex');
+    const agentsIdx = markers.indexOf('.agents');
+    const claudeIdx = markers.indexOf('.claude');
+    // All three must be present
+    expect(codexIdx).toBeGreaterThanOrEqual(0);
+    expect(agentsIdx).toBeGreaterThanOrEqual(0);
+    expect(claudeIdx).toBeGreaterThanOrEqual(0);
+    // .codex before .agents before .claude
+    expect(codexIdx).toBeLessThan(agentsIdx);
+    expect(agentsIdx).toBeLessThan(claudeIdx);
+  });
+
+  test('function signature accepts no arguments', () => {
+    // locateBinary should be callable with no arguments
+    expect(typeof locateBinary).toBe('function');
+    expect(locateBinary.length).toBe(0);
+  });
 });