feat: update notifier — warn when perchd is outdated

[irwansetiawan]

On each run, if the installed perchd is behind npm's latest, print a one-line notice:

⚠ Your perchd version 0.5.0 is outdated. Latest is 0.6.0.
  Update with: npm i -g perchd@latest

Design — never delay a command, never corrupt output

Mirrors update-notifier: the notice reflects the previous background check; the hot path is one small file read.

• Async, off the hot path. A stale cache (>24h) triggers a detached, unref'd perchd __update-check (hidden argv branch, handled before cac) that fetches the registry and writes $XDG_CACHE_HOME/perchd/update-check.json. The current command never awaits the network; the result shows next time.
• Off stdout — load-bearing. The notice goes to stderr, and the whole feature is skipped when stdout isn't a TTY. perchd path is read via $(...) in the shell cd function, so a notice on stdout would corrupt it. Verified: p=$(perchd path main) stays clean.
• Fail-silent + quiet where it should be. Offline / 404 / timeout leaves the cache untouched (no crash). Skipped under CI, NO_UPDATE_NOTIFIER=1, and pnpm dev (running from source).
• No new dependency — native fetch (Node 20) to registry.npmjs.org/perchd/latest.

Files

• src/core/update.ts — isNewer, updateNotice, cache read/write, isStale, runUpdateCheck (injectable fetcher), refreshInBackground, maybeNotifyUpdate, updateCheckSuppressed.
• src/cli.ts — hidden __update-check branch + one maybeNotifyUpdate(version) call.

Verification

• pnpm typecheck clean · pnpm test 135/135 (10 new unit tests).
• Drove the real binary end-to-end with a temp XDG_CACHE_HOME: real registry fetch writes the cache; a seeded "outdated" cache prints the notice (via pty); perchd path stdout stays clean; a stale cache triggers the detached refresh that advances checkedAt and pulls latest from the registry after the parent exits; and it correctly stays silent when you're ahead of latest.

Release

Additive, non-breaking. Filed under [Unreleased] in the CHANGELOG — no version bump here; it'll ride whatever you publish next (0.5.0 is still unpublished).