docs: add doctests to repos.py and localgit.py helpers (#80)

[tschm]

Fixes #80.

Done when: repos.py and localgit.py each have at least one doctest that pytest runs, and coverage stays at 100% of lines and branches.

Adds examples, which pytest runs through --doctest-modules, to four pure helpers:

• repos.resolve_path: ~/x, a relative path, bare ~, and an absolute path that doesn't exist all land under the /host mount
• repos._declared_forge: normalises case and whitespace, returns None when no forge is declared, and refuses an unknown forge with the exact FleetError text
• localgit._is_test: shows the test-file conventions (tests/, test_*.py, _test.go, .test.ts) and two near misses (contest.py, testing_utils.py)
• localgit._fingerprint: keeps the mtime to sub-second precision, the property its comment says matters

_is_test had no docstring until now. No code changed.

Gates

• prek run --all-files: all hooks pass.
• mypy (strict): clean.
• pytest: 465 passed (461 plus 4 doctest blocks), 100% of lines and branches.

🤖 Generated with Claude Code

Closes #80