Fix pre-existing ruff and mypy strict-mode violations

[severity1]

Summary

Cleans up 12 pre-existing lint/type errors on main that were blocking uv run ruff check . and uv run mypy scripts/ from returning clean:

• ruff (1): E402 in tests/test_trigger.py - import importlib sat below a sys.path.insert call. importlib is stdlib and has no path dependency, so it moves cleanly to the top-of-file imports block.
• mypy strict (11): Missing generic parameters on dict, no-any-return on json.load, untyped main() functions, and an overly loose return type on handle_git_commit.

These existed on main before any recent PRs - verified via git stash + baseline run. No runtime behaviour changes in this PR.

Why bother

Baseline noise in lint/type output makes it hard to tell real issues from accepted ones in future PRs. With the baseline clean, any new violation stands out immediately instead of being hidden in a pile of 12 pre-existing errors. This is groundwork for eventually adding these checks to a pre-commit hook or GitHub Action.

Changes

File	Change
scripts/trigger.py	Import typing.Any; dict -> dict[str, Any] on load_config, handle_stop, handle_pre_tool_use; annotate main() -> None; typed local for stdin parse
scripts/post-tool-use.py	Same typing.Any + dict[str, Any] treatment on load_config and main(); narrow handle_git_commit return to dict[str, str] | None matching the real {"hash", "message"} shape
tests/test_trigger.py	Move import importlib to top-level imports

Verification

$ uv run ruff check .
All checks passed!

$ uv run ruff format --check .
7 files already formatted

$ uv run mypy scripts/
Success: no issues found in 2 source files

$ uv run pytest tests/
============================= 123 passed in 0.90s ==============================

Test plan

• uv run ruff check . clean
• uv run ruff format --check . clean
• uv run mypy scripts/ clean
• uv run pytest tests/ 123/123 passing
• No runtime behaviour changes (only type annotations and import order)

Relationship to #27

Independent cleanup PR. #27 fixes bugs; this one fixes the baseline. Merging either first works; if #27 merges first, a trivial rebase may be needed here.

[severity1]

Absorbed into PR #27 (fix/subagent-stop-cleanup-and-windows-paths). All type annotation changes from this branch have been merged there directly. 132 tests passing, ruff and mypy clean.