feat: dynamic execution mode — Undecided tasks, two-phase dispatch, assign API

- ExecutionMode enum adds Undecided variant (default for new tasks)
- Webhook creates tasks as Undecided instead of hardcoded SshCli
- Dispatch loop: Phase 1 matches ssh_cli hosts, Phase 2 marks remaining as HttpPull
- Dequeue now returns http_pull AND undecided tasks (atomic claim)
- New endpoint: POST /api/v1/tasks/{id}/assign for coordinator explicit assignment
- Backward compatible: existing SshCli/HttpPull tasks unaffected
- 37 tests passing (6 new)

openspec/changes/dynamic-execution-mode/.openspec.yaml

@@ -0,0 +1,2 @@
+schema: spec-driven
+created: 2026-05-12