feat: combine workspace backend lifecycle and SCM repo discovery

[neversettle17-101]

Summary

• combines backend workspace project session materialization from feat: materialize workspace project sessions #2224
• combines workspace project lifecycle teardown from feat: add workspace project lifecycle teardown #2243
• adds SCM observer support for workspace child repositories so session branches/PRs from nested workspace repos are discovered and tracked
• keeps workspace session worktree rows available until teardown has inspected them, while still clearing restore markers after clean kill

Validation

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

[neversettle17-101]

Split this oversized PR into two focused PRs:

• feat: materialize workspace project sessions #2327: workspace project backend lifecycle/materialization/default-branch metadata
• feat: add workspace lifecycle and SCM follow-up #2328: stacked SCM observer support for workspace child repos

Leaving this PR as superseded by the split stack.