Add Command Code integration to spec-kit (#4019)

* Add Command Code integration to spec-kit

Adds `command-code` as a built-in skills-based integration so Spec Kit can
be installed into Command Code. Command Code loads agent skills from
`.commandcode/skills/speckit-<name>/SKILL.md` and invokes them in chat as
`$speckit-<command>`.

- New `CommandCodeIntegration` (SkillsIntegration) writing to
  `.commandcode/skills/`; declared multi-install safe (static, isolated
  agent root).
- Register in `_register_builtins()` and the integration catalog.
- Add `command-code` to `DOLLAR_SKILLS_AGENTS` so next-steps guidance
  renders `$speckit-*` invocations.
- Tests: reuse `SkillsIntegrationTests` mixin plus a dollar-invocation
  next-steps test; registry completeness updated.
- Docs: README and docs/reference/integrations.md (supported agents +
  multi-install-safe table).

Co-authored-by: CommandCodeBot <noreply@commandcode.ai>
Assisted-by: Command Code (autonomous)

* Fix issue template agent lists to include command-code

The runtime AGENT_CONFIG now includes command-code, but the GitHub issue
templates and the consistency test's expected key list were not updated,
failing test_issue_template_agent_lists_match_runtime_integrations.

Co-authored-by: CommandCodeBot <noreply@commandcode.ai>
Assisted-by: Command Code (autonomous)

---------

Co-authored-by: CommandCodeBot <noreply@commandcode.ai>

src/specify_cli/_invocation_style.py

@@ -9,7 +9,7 @@ and ``specify init``'s next-steps output stay consistent.
 from __future__ import annotations
 
 # Agents that render $speckit-<name> (chat invocation) when in skills mode.
-DOLLAR_SKILLS_AGENTS: frozenset[str] = frozenset({"codex", "zcode"})
+DOLLAR_SKILLS_AGENTS: frozenset[str] = frozenset({"codex", "zcode", "command-code"})
 
 # Agents that always render /speckit-<name>, regardless of ai_skills.
 ALWAYS_SLASH_AGENTS: frozenset[str] = frozenset({"devin", "droid", "grok", "trae", "zed"})