feat: materialize workspace project sessions

[neversettle17-101]

Summary

• split the smallest workspace backend core out of feat: combine workspace backend lifecycle and SCM repo discovery #2322
• add the workspace project port/config shapes and gitworktree materialization for root plus child repo worktrees
• wire session spawn to create workspace project worktrees and record their session worktree rows
• keep rollback/seed-row handling self-contained so this PR passes independently

Testing

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

Split from #2322. References #2222.