chore(lefthook): add CI-style path filtering to hooks

[wpfleger96]

Every lefthook command ran unconditionally on every commit/push, regardless of which surface the change touched. This meant a rust-only push always ran flutter analyze, a mobile-only push always ran cargo test, etc.

Add glob: (and exclude: for desktop vs desktop-tauri) to each command so lefthook skips commands whose surface the push diff doesn't touch. The mechanism is lefthook's built-in push-range filtering (git diff --name-only HEAD @{push}) for pre-push and staged-file filtering for pre-commit — the same information CI's dorny/paths-filter changes job uses for PR diffs.

What changed

pre-commit:
  rust-fmt          → glob: ["crates/**", "examples/countdown-bot/**"]
  desktop-tauri-fmt → glob: ["desktop/src-tauri/**"]
  desktop-fix       → glob: ["desktop/**", "pnpm-lock.yaml"], exclude: ["desktop/src-tauri/**"]
  web-fix           → glob: ["web/**", "pnpm-lock.yaml"]
  mobile-fix        → glob: ["mobile/**"]

pre-push:
  branch-skew        → no glob (always runs)
  rust-tests         → glob: ["crates/**", "migrations/**", "schema/**", "Cargo.toml", ...]
  desktop-check      → glob: ["desktop/**", "pnpm-lock.yaml"], exclude: ["desktop/src-tauri/**"]
  desktop-test       → glob: ["desktop/**", "pnpm-lock.yaml"], exclude: ["desktop/src-tauri/**"]
  desktop-tauri-test → glob: ["desktop/src-tauri/**"] UNION [rust filter] (mirrors ci.yml:113)
  mobile-test        → glob: ["mobile/**"]

A header comment in lefthook.yml states that these globs mirror ci.yml's changes job filters and must be kept in sync.

Deliberate deviations from ci.yml

1. Workflow-file path omitted — CI adds .github/workflows/ci.yml to its rust/mobile filters; a CI-config-only edit doesn't need a local test run.
2. desktop-check/desktop-test don't union rust — CI's Desktop Core job triggers on rust OR desktop-rust, but those commands are pure TS (biome + node:test) with no Rust dependency. Triggering them on rust changes would be spurious locally.
3. Deletion-only pushes don't trigger local hooks — lefthook 2.1.x drops deleted paths from push-file discovery (extractFiles existence check in repository.go). CI's dorny/paths-filter remains the gate for deletion-only surface changes. Deliberate: accepted, not worked around.

Semantics to note

lefthook pre-push diffs against the remote branch tip (HEAD @{push}), not the full PR history. This means a later unrelated push won't re-run an earlier surface's tests even if that surface has commits earlier in the branch. This is coherent — each surface was gated when it was pushed — but it differs from CI which diffs the whole PR against main.

Verification matrix (LEFTHOOK_VERBOSE=1)

Scenario	Runs	Skips
crates/buzz-core/src/lib.rs	rust-tests, desktop-tauri-test ✅	desktop-check, desktop-test, mobile-test — all "no matching push files"
mobile/lib/main.dart	mobile-test ✅	rust-tests, desktop-tauri-test, desktop-check, desktop-test — all "no matching push files"
desktop/src/App.ts	desktop-check, desktop-test ✅	rust-tests, desktop-tauri-test, mobile-test — all "no matching push files"
README.md (any file)	branch-skew ✅	everything else skipped

Skip message in each case: (skip) no matching push files