feat(git-sync): add reusable scheduled repository sync service

[Bad3r]

Summary

Create one reusable, conflict-safe scheduled git-sync primitive for user data repositories, then configure Logseq and Obsidian as consumers of that primitive instead of solving the same sync lifecycle twice.

Child Issues

• chore: automate Logseq graph git sync #104
• chore: automate Obsidian vault git sync #105

Goals

• Provide one implementation for scheduled repository sync.
• Keep force-push and overwrite behavior impossible by default.
• Handle no-change runs without empty commits.
• Make failures observable through logs and notifications.
• Support one-command manual sync for each configured repository.
• Keep application-specific paths, remotes, credentials, and ignore rules in per-app adapters.

Acceptance Criteria

• Reusable sync script/service design exists.
• Scheduled trigger exists for configured repositories.
• Pull/rebase/push behavior is conflict-safe and never force-overwrites remote history.
• No-change runs exit cleanly without commits.
• Logs include changed-file summary, commit hash when applicable, and push result.
• Manual sync command exists.
• Logseq adapter is wired through chore: automate Logseq graph git sync #104.
• Obsidian adapter is wired through chore: automate Obsidian vault git sync #105.
• Documentation covers setup, scheduling, credentials, recovery, and troubleshooting.

Notes

This is the parent tracker. Logseq and Obsidian issues should stay focused on application-specific configuration after the shared sync primitive exists.