feat: gitignore .mcp.json on cce init

[fazleelahhee]

Summary

`.mcp.json` carries an absolute path to the `cce` binary and a
`--project-dir` argument — both differ on every contributor's
machine. Committing it forces everyone to share one path layout, which
never holds in practice (homedir paths, pipx vs uv vs pip installs,
etc.).

`cce init` regenerates `.mcp.json` on every machine, so the right
default is to keep it out of version control alongside the other
per-machine entries (`.cce/`, `.claude/settings.local.json`).

Behaviour

• Newly-init'd projects get `.mcp.json` in the managed CCE block of
  `.gitignore`.
• Existing projects pick up the new entry on the next `cce init`
  (`ensure_gitignore` is idempotent — it only appends missing
  entries, never rewrites unrelated content).
• Projects that already explicitly listed `.mcp.json` don't get a
  duplicate.
• Existing tracked `.mcp.json` files are not auto-untracked;
  that's a manual `git rm --cached` decision the user makes.

Tests

• `test_ignores_mcp_json`: `ensure_gitignore` writes `.mcp.json`
  into a fresh `.gitignore`.
• `test_mcp_json_not_duplicated_if_user_already_ignored`:
  pre-existing entry is preserved without duplication.

Verification

• `pytest tests/test_project_commands.py` → 60 passed
• `ruff check` → clean

Test plan

• `cce init` on a fresh repo — `.mcp.json` appears in
  `.gitignore` under the CCE-managed block
• `cce init` on an existing repo that already has
  `.mcp.json` listed — no duplicate entry
• `cce init` on a repo with a previously-tracked
  `.mcp.json` — file stays tracked (the user-driven
  `git rm --cached .mcp.json` step is intentional)

[rajkumarsakthivel]

LGTM. Correct change, well-scoped.

.mcp.json has absolute paths that differ per machine, and cce init regenerates it. Gitignoring it is the right default. Tests cover both the fresh write and the dedup case. No breaking change for repos that already track it.