Add tests for Write-on-existing-file and unknown tool name paths in resolveAction

[gregology]

Closes #17

resolveAction has three branches: the toolToAction map lookup, the Write tool special case (which checks os.Stat to pick between create and edit), and the fallback that returns ActionAll for unknown tools. Only the Write-as-create path had test coverage.

This adds two tests:

• TestHandleClaudeHook_WriteExistingFile — Write targeting a file that already exists on disk. Verifies that os.Stat succeeds and we get ActionEdit, so edit-scoped context shows up and create-scoped context doesn't.
• TestHandleClaudeHook_UnknownToolName — An unrecognized tool name like "UnknownTool" hits the ActionAll fallback. Verifies that entries with on: all match while entries with specific on values (like on: edit) don't.

The issue also suggested testing Write with an unresolvable path (permission denied on stat). I skipped that one — it's basically the same as the existing TestHandleClaudeHook_WriteNewFile since any os.Stat error falls through to ActionCreate. Manufacturing a reliable permission-denied scenario in a cross-platform test felt like more trouble than the coverage was worth.

Stuck with individual test functions rather than converting to table-driven. The existing tests each need slightly different setup (different AGENTS.yaml content, different file states), so a shared table would mostly just add indirection without reducing code.

Resolves #17