fix(cli): scope metadata db path to target repository

[AperturePlus]

Motivation

• Prevent .aci/index.db from being created relative to the caller's CWD when running aci index /path/to/repo, which caused aci search to report "Path has not been indexed" when the search was run from the project directory.

Description

• Added a helper _project_metadata_db_path(path: Path) -> Path to derive the project-scoped metadata DB path as <repo>/.aci/index.db.
• Updated get_services() to accept an optional metadata_db_path and forward it to create_services() so services initialize using the intended metadata file.
• Wired CLI commands index, search (when given --path or implicit project), and update to call get_services(metadata_db_path=_project_metadata_db_path(...)), ensuring metadata operations are scoped to the target repository instead of CWD.
• Design rationale: chose an explicit, minimal change to pass a project-scoped metadata path into service initialization for low-risk correctness; the alternative of falling back to the global registry was considered but would be a broader behavioral change and was deferred.

Testing

• Ran linting with uv run ruff check src tests and it passed successfully (✅).
• Executed targeted unit tests with uv run pytest tests/unit/test_cli.py tests/unit/test_cli_metadata_db_path.py -q and they passed (✅).
• Ran full test suite with uv run pytest tests/ -v --tb=short -q --durations=10 but this run was interrupted after exposing pre-existing unrelated failures in the broader suite (⚠️); the failures are not caused by this change.
• Ran type checks with uv run mypy src --ignore-missing-imports --no-error-summary which reported existing repository-wide typing issues unrelated to the CLI fix (⚠️).

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Codex Task