fix: ensure idempotent project-relative path rewriting in CommandRegistrar

[darion-yaphet]

Description

`CommandRegistrar.rewrite_project_relative_paths()` previously relied on three consecutive `str.replace` calls followed by three separate `re.sub` passes, and

used .replace(".specify/.specify/", ".specify/").replace(".specify.specify/", ".specify/") as an ad-hoc post-processing fix. This "patch-on-patch" design was
fragile, caused redundant string scanning, and risked regressions when paths were processed multiple times.

This PR refactors the path normalization into a single unified regex match callback:
1. **Single-pass regex callback**: Consolidates multiple string and regex passes into one regex scan with a match handler.
2. **Native idempotency guard**: If a path prefix is already `.specify/`, it is returned unchanged without modification, preventing duplicate prefixing from the

source and eliminating the need for subsequent .replace() corrections.
3. Deterministic routing: Parent-relative paths (../) consistently route to root .specify/<target>/, while top-level / relative scripts/ preserve
extension-local scoping when extension_id is supplied.
4. Expanded delimiters: Recognizes common Markdown punctuation enclosures such as brackets [], parentheses (), braces {}, angle brackets <>, quotes,
and backticks.

Testing

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- [x] Ran existing tests with `pytest tests/test_extensions.py` (538 passed)
- [x] Ran new idempotency and delimiter test suite with `pytest tests/test_extensions.py -k "test_rewrite_project_relative_paths"` (5 passed)
- [x] Ran linter check with `ruff check src/specify_cli/agents.py tests/test_extensions.py` (0 errors)

## AI Disclosure

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- [ ] I **did not** use AI assistance for this contribution
- [x] I **did** use AI assistance (describe below)