fix: make hook registration idempotent

[jcfischer]

Summary

• make hook registration replace existing matching registrations instead of append-skipping
• reconcile legacy untagged duplicate hook entries with the same matcher and command
• add regression coverage for the duplicate PreToolUse Bash hook shape from arc install/upgrade re-appends provides.hooks non-idempotently → duplicate PreToolUse registrations #183

Fixes #183.

Verification

• rtk bun test test/unit/hooks.test.ts
• rtk bun test test/commands/remove-hooks-137.test.ts
• rtk bun test test/commands/install.test.ts
• rtk bunx tsc --noEmit
• rtk bun run lint
• rtk bun test

[jcfischer]

Review: CodeQuality + Architecture

Lenses applied: CodeQuality, Architecture

Analysis

Clean, well-scoped fix for #183 (duplicate hook registrations from arc install/upgrade).

Core change: registerHooks shifts from append-with-skip to filter-then-push (replace-by-tag). Three small helpers (matcherKey, hookGroupHasCommand, shouldReplaceHookGroup) keep the predicate readable and testable.

Correctness verified:

• shouldReplaceHookGroup matches on (matcher, command, pkg) — replaces same-pkg entries AND legacy untagged entries (_pai_pkg === undefined), preserves entries from other packages. ✓
• Existing "deduplicates identical hook commands" test still holds: filter removes the old entry, push adds fresh → net 1. Idempotent. ✓
• New test covers the exact scenario from #183: multiple untagged duplicates + one tagged + different matcher + different package → collapsed to 3 entries (OtherPkg/Bash, Cortex/Bash, Read). ✓
• matcherKey normalizes undefined → "" via nullish coalescing (also handles null from malformed JSON). ✓

Minor behavior change (acceptable): Old code was a no-op on re-register (skip if found). New code always removes-then-pushes, rewriting the entry. End state is identical; settings.json gets touched on every install even when unchanged. Not a problem — arc install already rewrites settings.json unconditionally.

No findings. blockers=0 majors=0 nits=0 — recommend: approve ✅