Prepare SourceBraid first-release identities

[patrickschiller]

What changed

• Adds RELEASING.md with the reproducible public release process.
• Replaces all remaining Stowmark iOS bundle, App Group, Keychain, and test
  identifiers with SourceBraid identities.
• Updates the iOS setup documentation for a first App Store publication.
• Links the release guide from the English and German READMEs.

Why

SourceBraid has not yet been published to the Chrome Web Store, ChatGPT/Codex
directory, TestFlight, or App Store. The public project incorrectly assumed an
existing pre-rebrand iOS release and did not contain the promised public release
guide.

Privacy, permissions, and migration impact

• No browser permissions, GitHub scopes, network endpoints, archive formats,
  deletion behavior, or credential handling changed.
• The iOS bundle IDs become de.patrickschiller.sourcebraid and
  de.patrickschiller.sourcebraid.share.
• The App Group and Keychain namespace move to SourceBraid. Because no iOS build
  has been published, there is no installed-user migration or existing store
  identity to preserve.
• The public release guide explicitly excludes private repository history,
  website/, web-clips/, credentials, caches, and local configuration.

Validation

• node --test tests/capture-utils.test.js — 15 passed
• python3 -m unittest discover -s tests -p "test_*.py" — 34 passed
• Unsigned iOS simulator build — succeeded
• Chrome allowlist package build — succeeded
• Skills-only plugin package build — succeeded
• App and Share Extension plist/entitlement lint — passed
• git diff --cached --check
• Verified the public tree contains no Stowmark or Stomark references