refactor: extract Astrolabe to separate repository

Astrolabe has been extracted to its own repository at
github.com/cbcoutinho/astrolabe for independent releases.

Changes:
- Replace third_party/astrolabe/ directory with git submodule
- Remove astrolabe-ci.yml and appstore-build-publish.yml workflows
- Remove scripts/bump-astrolabe.sh
- Remove Astrolabe sections from bump-version.yml workflow
- Remove Astrolabe build steps from test.yml CI workflow
- Remove astrolabe volume mount from docker-compose.yml
- Simplify astrolabe install hook to always use app store
- Update CONTRIBUTING.md to reflect two-component monorepo

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitmodules

@@ -4,3 +4,6 @@
 [submodule "third_party/notes"]
 	path = third_party/notes
 	url = https://github.com/cbcoutinho/notes
+[submodule "third_party/astrolabe"]
+	path = third_party/astrolabe
+	url = git@github.com:cbcoutinho/astrolabe.git