feat(skill): add the agent-skill install layer and harden the packaged skill

Ship `skills/codeman` as an installable Claude Code skill rather than a
repo-only reference, and fix six defects found while verifying it live.

Install layer:
- `codeman skill install [--case <name>]` / `codeman skill uninstall`.
  Case names resolve through linked-cases.json first, mirroring the
  server's resolveCasePath(), so a case linked in from outside
  ~/codeman-cases no longer fails with "Case not found".
- applyAgentSkill() / installAgentSkillInto() / removeAgentSkillFrom() in
  hooks-config.ts. Copies are marker-owned, so an unmarked user-authored
  skill is never touched, and a symlinked skill dir is refused (this
  repo's own .claude/skills/codeman is a symlink to the source).
- Synced `agentSkillEnabled` setting, default OFF: schemas.ts,
  ports/config-port.ts, server.ts, session-routes.ts (add-only injection
  on Claude session create and quick-start), plus the App Settings toggle.

Skill content fixes, each reproduced before and after:
- Fail-closed `delete_session` replaces `is_self ... || curl -X DELETE`.
  Shell state does not survive between agent tool calls, and an undefined
  is_self exited 127, firing the `||` branch and deleting the caller's own
  session with the one guard bypassed. The request now lives inside the
  guard, so a lost preamble deletes nothing.
- clientId is a fixed literal instead of `agent-$$`. The pid changes per
  tool call, so the documented resend-identical-request loop stopped being
  a duplicate and retyped the prompt, submitting the turn twice.
- `last-response` is now the documented read path for claude and codex
  workers. It returns clean transcript text; the terminal scrape it
  replaces returns a wall of TUI repaint noise. Its transcript flush lags
  the stop signal, so the recipes poll it rather than reading once.
- quick-start examples branch on `.success`. Previously a failed spawn
  yielded the literal session id "null" and burned the whole readiness
  budget before reporting jq noise instead of the cause.
- Documented that turning `agentSkillEnabled` off sweeps nothing, and
  corrected the hooks-config comment that claimed a toggle-off sweep
  exists. Per-case cleanup is `codeman skill uninstall --case <name>`.
- Documented that SESSION_BUSY means the 50-session cap on quick-start,
  and that caseName resolves linked cases, so a generic name can land a
  worker in a real repo.

Tests: test/agent-skill.test.ts covers install, refresh, idempotence,
marker ownership and symlink refusal against the real packaged source;
test/quick-start.test.ts covers injection behind the setting.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/web/routes/session-routes.ts

@@ -79,6 +79,7 @@ import {
   updateCaseModel,
   stripCaseEnvKeys,
   applyStatusLineConfig,
+  applyAgentSkill,
   refreshStaleCodemanHooks,
 } from '../../hooks-config.js';
 import { generateClaudeMd } from '../../templates/claude-md.js';
@@ -699,6 +700,13 @@ export function registerSessionRoutes(
     // cases (writeHooksConfig already wrote the secret) and for non-Codeman/absent hooks.
     if ((body.mode ?? 'claude') === 'claude') {
       await refreshStaleCodemanHooks(workingDir).catch(() => {});
+      // Agent skill (docs/agent-control-plan.md §2): ADD-ONLY on create, same shared-
+      // .claude rationale as the statusLine above: a create must never remove the
+      // skill from under other live sessions in the repo. Marker-guarded, so a
+      // user's own skills/codeman is never touched.
+      if (await ctx.getAgentSkillEnabled()) {
+        await applyAgentSkill(workingDir, true).catch(() => {});
+      }
     }
 
     // Check OpenCode availability if requested
@@ -2766,6 +2774,15 @@ export function registerSessionRoutes(
       await refreshStaleCodemanHooks(resolvedCasePath).catch(() => {});
     }
 
+    // Agent skill injection (docs/agent-control-plan.md §2): ADD-ONLY on create,
+    // marker-guarded (a user's own skills/codeman is never touched). Claude mode only
+    // (`.claude/skills/` is a Claude Code surface); skipped for remote cases, whose
+    // casePath lives on another host. Docker cases qualify: hostWorkspacePath is a
+    // real host dir and the skill crosses the bind mount like the rest of `.claude/`.
+    if (!remote && mode === 'claude' && (await ctx.getAgentSkillEnabled())) {
+      await applyAgentSkill(resolvedCasePath, true).catch(() => {});
+    }
+
     // Docker cases: the workspace is a REAL host dir bind-mounted into the container.
     // Scaffold hooks (+ a CLAUDE.md) if MISSING so in-container permission prompts and
     // hook-idle detection fire (decision: wire hooks now). Never clobbers an existing