docs: record Homebrew receipt ownership

[tarnish233]

Summary

Update the repository guide from the pre-release instruction (“remove auto_updates true later”) to the post-release invariant:

• GitPic 0.20.10 is the first release that always hands cask-managed updates back to Homebrew.
• The release DMG and tap SHA-256 were verified to match before the stanza was removed in tarnish233/homebrew-tap#2.
• auto_updates true must not be re-added because the cask-managed artifact no longer self-updates; Homebrew receipt comparison is intentional.
• Keep the one-release 0.20.9 receipt transition documented, and keep uninstall quit: load-bearing.

Verification

• git diff --check
• documentation-only change

Corresponding tap change: tarnish233/homebrew-tap#2