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)

docs/agent-api-reference.md

@@ -236,7 +236,7 @@ curl http://FLEET_API_URL:PORT/api/v1/tasks/org%2Frepo%2342
 POST /api/v1/tasks/dequeue
 ```
 
-Requires Bearer token if `http_pull_token` is configured. Only returns tasks with `execution_mode = http_pull`.
+Requires Bearer token if `http_pull_token` is configured. Returns tasks with `execution_mode = http_pull` and may also claim `undecided` tasks, atomically converting them to `http_pull` when dequeued.
 
 **Request:**
 
@@ -258,6 +258,33 @@ curl -X POST http://FLEET_API_URL:PORT/api/v1/tasks/dequeue \
 
 ---
 
+### Assign Task
+
+```
+POST /api/v1/tasks/{task_id}/assign
+```
+
+Explicitly assign a created task to a specific agent or host.
+
+**Request:**
+
+| Field | Type | Required | Description |
+|-------|------|----------|-------------|
+| agent_id | string | yes | Registered http_pull agent ID or configured ssh_cli host ID |
+| execution_mode | string | no | Optional override: `ssh_cli` or `http_pull`. If omitted, the server auto-detects it from the target. |
+
+**Response:** `200 OK` — updated [Task](#task-object).
+
+**Errors:** `404` if task or agent not found. `400` if task status is not assignable.
+
+```bash
+curl -X POST http://FLEET_API_URL:PORT/api/v1/tasks/org%2Frepo%2342/assign \
+  -H 'Content-Type: application/json' \
+  -d '{"agent_id": "worker-03"}'
+```
+
+---
+
 ### Update Task Status (http_pull only)
 
 ```
@@ -455,7 +482,7 @@ Receives Forgejo webhook events. Requires HMAC-SHA256 signature header.
 
 **Priority values:** `low`, `normal`, `high`, `urgent`
 
-**Execution mode values:** `ssh_cli`, `http_pull`
+**Execution mode values:** `undecided`, `ssh_cli`, `http_pull`
 
 ### Receipt Object