feat(agent-skill): turn the Codeman agent skill on by default

An absent `agentSkillEnabled` now means on, on the server
(`getAgentSkillEnabled()` reads `!== false`, like `workspaceHooksEnabled`)
and on the page (the App Settings switch loads `!== false`, so a Save of any
other setting no longer writes the old default back). Only an explicit false
turns it off. A partial settings PUT that omits the key leaves it as stored,
since the route merges the request into the file.

The switch's help text says it is on by default, that the copy stays out of
git, and that turning it off removes no copy already written. The shared
route-test mock now defaults to on too, so route tests see what a user sees.

Tests: quick-start against a real server (absent key injects, a partial PUT
keeps it on, an explicit false stays off through a later partial PUT), the
real openAppSettings/saveAppSettings in JSDOM (absent shows on, explicit false
shows off, a Save on a never-touched install sends true and passes the
schema), and the plain create route at the mock's default.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.changeset/agent-skill-default-on.md

@@ -0,0 +1,5 @@
+---
+'aicodeman': minor
+---
+
+**The Codeman agent skill is on by default.** New sessions get the skill in their folder, where their CLI looks for skills (`.claude/skills/codeman/` for Claude Code, `.agents/skills/codeman/` for Codex, Gemini CLI, OpenCode, Antigravity, Grok, DeepSeek and OMP; still none for Pi or a plain shell), so an agent can start and drive workers without an install step. The copy now ignores itself in git (a `.gitignore` of `*` inside the copy, written once and never overwritten), so it never shows up as a new file in your repository, and `codeman skill uninstall` removes it with the copy. A `settings.json` without the `agentSkillEnabled` key now means on; an explicit `false` stays off, which includes every install whose App Settings were saved on an earlier version, since that save wrote the old default. Turn it off under App Settings, Agents & CLIs, All agent CLIs, **Codeman Agent Skill**. Turning it off still removes no copy already written: `codeman skill uninstall --case <name>` does that per case.