feat: add workspace lifecycle and SCM follow-up

[neversettle17-101]

Summary

• stacked follow-up on feat: materialize workspace project sessions #2327 with the heavier workspace backend changes moved out of the core PR
• add workspace project teardown/restore, dirty preservation, cleanup behavior, and default-branch metadata
• add SCM observer support for workspace child repositories

Testing

• cd backend && go test ./internal/observe/scm ./internal/session_manager ./internal/adapters/workspace/gitworktree ./internal/storage/sqlite/store ./internal/service/project

Stacked on #2327. Split from #2322. References #2222.

[neversettle17-101]

Requesting changes. Restore now ignores legacy saved worktree rows, so sessions saved before this state transition change can be stranded after upgrade.