docs: add update-check spec and implementation plan (#568)

* docs: add update-check spec and implementation plan

These files were created during feature/cli-update-check (#566) but
were never committed before the branch merged.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* fix: address review feedback on update-check plan

- Fix critical ordering: check for updates and confirm with user
  BEFORE calling UpdateAsync (which spawns detached process)
- Fix "already up to date" check to be channel-aware — pre-release
  users with only stable updates no longer incorrectly proceed
- Fix misleading commit message: s/Process.MainModule/RuntimeEnvironment/

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: joshsmithxrm