feat(cli): update notifications + self-update (strix --update)

[bearsyankees]

No description provided.

[greptile-apps]

Greptile Summary

This PR adds update notifications and a strix --update self-update command to the CLI. A background daemon thread fetches the latest version once per 24 h, caches the result in ~/.strix/update-check.json, and surfaces a Rich-formatted notice (or an interactive prompt for binary installs) at startup and after scan completion.

• update_check.py handles version fetching (GitHub Releases for binary installs, PyPI for package-manager installs), cache read/write, platform detection, archive download, binary replacement, and Windows-specific rename/rollback logic.
• main.py integrates --update, the background check, the pre-scan update prompt with os.execv re-exec on Unix, and a post-scan notify_update call.
• Tests cover version comparison edge cases, cache staleness, CI environment disabling, and the non-binary update path using an explicit io.StringIO console buffer.

Confidence Score: 5/5

Safe to merge; the update path handles errors, Windows rollback, and staged-file cleanup correctly.

The core self-update mechanics are implemented defensively. The two findings are minor UX gaps that don't affect correctness for x86-64 users.

strix/interface/update_check.py — specifically the supported set in _release_target() and the always-skipped checksum path in _fetch_asset_digest().

Important Files Changed

Filename	Overview
strix/interface/update_check.py	New module implementing background version checking, update notifications, and self-update for binary installs. Windows rollback and staged-file cleanup are handled correctly. linux-arm64 is missing from the supported-target set despite the aarch64→arm64 mapping being defined. _fetch_asset_digest reads a digest field that GitHub's Releases API does not provide, so checksum verification is always skipped.
strix/interface/main.py	Wires up --update, background check, and pre-scan update prompt. Logic is sound for Linux/macOS binary installs; on Windows, a successful pre-scan update falls through without a restart notice.
tests/test_update_check.py	Good unit-test coverage of version comparison, cache logic, CI disabling, and the non-binary self_update path using an explicit io.StringIO Console.

_{Reviews (2): Last reviewed commit: "fix(update): verify release checksum, cl..." | Re-trigger Greptile}

[bearsyankees]

@greptile