fix(installer): strip stale auto-sync hooks on install and uninstall (#278)

Pre-0.8 installers wrote `codegraph mark-dirty` / `sync-if-dirty` hooks
to Claude Code's settings.json. Both subcommands were removed from the
CLI, so the Stop hook fails every turn ("unknown command
'sync-if-dirty'"). The cleanup that once removed them was lost when the
installer moved to the per-target architecture.

Add cleanupLegacyHooks(), wired into both install (upgrades self-heal)
and uninstall (so the npm preuninstall step fully reverses a legacy
install). Surgical at the command level: only codegraph's own hook
entries are dropped, so unrelated hooks sharing a matcher group or event
(e.g. GitKraken's `gk ai hook run`) survive, and a settings.json with no
legacy hooks is left byte-for-byte untouched.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>