feat(alerts): embed task snapshot in reminder alert metadata

[madtank]

Summary

Task `e55be7c8` — CLI-side half of making task reminder alerts task-aware.

Today the reminder alert card surfaces raw implementation fields and truncates. The alert payload already carries `source_task_id`, but the frontend has to round-trip to `/tasks/{id}` at render time. This enriches the envelope with a compact task snapshot so the card can render task title / priority / status / assignee inline.

Changes

• `alerts.py`: extract `_agent_name_for` + `_fetch_task_snapshot` helpers (shared with existing `_resolve_target_from_task`)
• `alerts.py`: `_build_alert_metadata` accepts `task_snapshot` kwarg; embeds at `alert.task` and `card_payload.task`
• `reminders.py`: `_fire_policy` fetches snapshot once per fire and passes it through
• `tests`: positive case (snapshot embedded with all fields) + negative case (fetch failure still fires reminder with `source_task_id` link fallback)

Shape

`alert.task` / `card_payload.task`:

```jsonc
{
"id": "task-snap",
"title": "Ship delivery receipts",
"priority": "urgent",
"status": "in_progress",
"assignee_id": "agent-orion",
"assignee_name": "orion",
"creator_id": "agent-chatgpt",
"deadline": "2026-04-17T00:00:00Z"
}
```

`card_payload.resource_uri` remains `ui://tasks/{source_task_id}` — frontend's Open Task button links here.

Coordination

Task `e55be7c8` is frontend_sentinel's — they render the task block on the alert card + wire Open Task. This PR is the CLI-side payload enrichment, unblocking their slice so it doesn't need a per-card `tasks.get` round-trip.

Composes with PR #53 (ax alerts MVP). Targeting `feat/alerts-cli-mvp` to stack cleanly.

Validation

```
PYTEST_DISABLE_PLUGIN_AUTOLOAD=1 pytest tests/test_alerts_commands.py tests/test_reminders_commands.py
21 passed in 11.88s
```

Test plan

• Existing 19 alert + reminder tests still pass
• New test: reminder with source_task_id embeds full task snapshot
• New test: reminder with unreachable task still fires without snapshot (source_task_id link fallback)
• Frontend e55be7c8 slice consumes `alert.task` + `card_payload.task`

🤖 Generated with Claude Code