feat: squad update-check --json — expose cached update status for tooling #1170

Summary

Add a squad update-check --json CLI command that reads the existing update-check.json cache (already maintained by self-update.ts) and emits structured JSON. This gives editor extensions, coordinator instructions (Squad's own and downstream forks), CI scripts, and IDE tooling a clean interface to query update status without OS-specific cache path resolution.

Motivation

self-update.ts already maintains an OS-specific cache at:

• Windows: %APPDATA%\squad-cli\update-check.json
• Linux: ~/.config/squad-cli/update-check.json
• macOS: ~/Library/Application Support/squad-cli/update-check.json

Today, anything outside the CLI process that wants this information has to:

1. Replicate the OS-specific path logic
2. Read & parse the JSON file
3. Check TTL freshness
4. Handle the (no cache yet) case

A CLI command exposing this cleanly is much simpler for consumers.

Proposed behavior

$ squad update-check --json
{
  "current": "0.9.6-insider.2",
  "channel": "insider",
  "latest": "0.9.7-insider.1",
  "updateAvailable": true,
  "cacheAge": "PT2H15M",
  "checkedAt": "2026-05-26T12:00:00.000Z"
}

Non-JSON (default human format):

$ squad update-check
Current: 0.9.6-insider.2 (insider channel)
Latest:  0.9.7-insider.1
Update available. Run `squad upgrade` to install.

Flags

• --json — structured output
• --refresh — bypass cache and re-fetch (otherwise serves from existing cache)
• Exit code: 0 (up to date) or 1 (update available); 2 on transport failure

Side effects

• None unless --refresh is set
• Honors SQUAD_NO_UPDATE_CHECK=1 (exits 0 silently with empty JSON / empty stdout)

Acceptance

• squad update-check --json emits the schema above
• Reuses the existing update-check.json cache
• --refresh forces a re-fetch
• Exit codes documented and stable
• No new network calls without --refresh

Context

Filed during architecture review for downstream issue tamirdresher_microsoft/squad-squad#1 ("Coordinator-level in-chat upgrade UX"). The downstream coordinator currently has to resolve OS-specific cache paths via a node one-liner inside coordinator instructions — a CLI command would eliminate that fragility.