[BUG] GeminiClientAdapter ignores project_root/user_scope and writes MCP config under cwd

[imk1t]

Describe the bug
GeminiClientAdapter resolves .gemini/settings.json from os.getcwd() instead of the adapter's project_root / scope-aware path context. When callers instantiate the adapter with project_root=..., MCP config is skipped or written under the current process cwd rather than the target project. This breaks non-cwd installs and makes Gemini inconsistent with other repo-local MCP adapters.

To Reproduce
Steps to reproduce the behavior:

1. From a checkout that is not the target project, run:

   from pathlib import Path
   from tempfile import TemporaryDirectory
   from apm_cli.adapters.client.gemini import GeminiClientAdapter
   
   with TemporaryDirectory() as tmp:
       root = Path(tmp)
       (root / ".gemini").mkdir()
       adapter = GeminiClientAdapter(project_root=root)
       adapter.update_config({"srv": {"command": "node"}})
   
       print("expected exists:", (root / ".gemini" / "settings.json").exists())
       print("actual adapter path:", adapter.get_config_path())

2. Observe that expected exists is False.
3. Observe that get_config_path() points at <cwd>/.gemini/settings.json.

Expected behavior
GeminiClientAdapter(project_root=root) should read and write root/.gemini/settings.json. If user scope is supported, user-scope behavior should be explicit and consistent with the target profile rather than silently using cwd.

Environment (please complete the following information):

• OS: Darwin 25.4.0 arm64
• Python Version: 3.12.12
• APM Version: 0.13.0 (9020aca)
• VSCode Version (if relevant): N/A

Logs
Current code path:

• src/apm_cli/adapters/client/gemini.py uses Path(os.getcwd()) in get_config_path().
• update_config() also gates on Path(os.getcwd()) / ".gemini".

Observed local repro:

expected_exists False
adapter_path /Users/ko1/.codex/worktrees/7541/apm/.gemini/settings.json

Additional context
This appears separate from #1266, which covers Gemini stdio env placeholder resolution, and #1062, which covered Gemini opt-in behavior. This issue is specifically about path/scope routing: adapter construction accepts project_root, but Gemini ignores it.