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/public/index.html

@@ -1637,6 +1637,14 @@
               </label>
               <span class="form-hint">Enable experimental Agent Teams for all new Claude sessions (disabled by default)</span>
             </div>
+            <div class="form-row form-row-switch">
+              <label>Agent Skill</label>
+              <label class="switch">
+                <input type="checkbox" id="appSettingsAgentSkill">
+                <span class="slider"></span>
+              </label>
+              <span class="form-hint">Give new Claude sessions the Codeman skill (start workers, send prompts, wait for results via the API)</span>
+            </div>
             <div class="form-row">
               <label>Claude Model</label>
               <select id="appSettingsClaudeModel" class="form-select">

src/web/public/settings-ui.js

@@ -385,6 +385,7 @@ Object.assign(CodemanApp.prototype, {
     this._applyCodexSettingsVisibility();
     // Claude Permissions settings
     document.getElementById('appSettingsAgentTeams').checked = settings.agentTeamsEnabled ?? false;
+    document.getElementById('appSettingsAgentSkill').checked = settings.agentSkillEnabled ?? false;
     document.getElementById('appSettingsClaudeModel').value = settings.claudeModel ?? '';
     document.getElementById('appSettingsOpusContext1m').checked = settings.opusContext1mEnabled ?? false;
     document.getElementById('appSettingsRemoteAutoReconnect').checked = settings.remoteAutoReconnect ?? true;
@@ -1553,6 +1554,7 @@ Object.assign(CodemanApp.prototype, {
       codexAnimationsEnabled: document.getElementById('appSettingsCodexAnimations').checked,
       // Claude Permissions settings
       agentTeamsEnabled: document.getElementById('appSettingsAgentTeams').checked,
+      agentSkillEnabled: document.getElementById('appSettingsAgentSkill').checked,
       claudeModel: document.getElementById('appSettingsClaudeModel').value,
       opusContext1mEnabled: document.getElementById('appSettingsOpusContext1m').checked,
       remoteAutoReconnect: document.getElementById('appSettingsRemoteAutoReconnect').checked,