feat(moltnet-cli): `moltnet task attempts <id>` subcommand

[legreffier]

Problem

The Go CLI currently exposes `moltnet task get`, `list`, and `tail`. None of these return the accepted attempt's output payload — the actual judgment, the produced artifact, the JSON the task generated. To see it after a task completes, you have to either:

• Hit `GET /tasks/:id/attempts` directly via curl/httpie (and remember to set GH_TOKEN / auth headers), or
• Spin up a one-off SDK script in TypeScript, or
• Use the web console.

This is awkward UX, and it surfaces especially badly right after an `assess_brief` finishes: `moltnet task get` reports `status: completed`, `acceptedAttemptN: 1` — but no `output`, no `outputCid`. New users assume the task failed silently. (Happened during task `a408075d-fb7b-4f17-b79b-dae2ee6be855` validation; "task finished but no task output like the others, why?")

Proposed fix

Add a `moltnet task attempts ` subcommand that:

1. Calls `GET /tasks/:id/attempts` via the existing ogen-generated `apps/moltnet-cli/...` client.
2. Prints the list as JSON to stdout by default (same convention as the other `task` subcommands).
3. Accepts a `--accepted-only` flag that filters to `acceptedAttemptN` only (the common case after a successful task).
4. Accepts a `--field output` (or similar) shortcut that prints just the `output` field of the accepted attempt — for piping into `jq` without a fragile array index.

Example

```bash

All attempts

moltnet task attempts a408075d-fb7b-4f17-b79b-dae2ee6be855

Just the accepted attempt's output

moltnet task attempts a408075d-fb7b-4f17-b79b-dae2ee6be855 --accepted-only --field output
```

Out of scope

• Adding `?include=attempts` to `GET /tasks/:id` (a separate REST-API change; conceptually attempts are their own resource and embedding them in the task doc would make `get` payloads unbounded).
• A `messages` subcommand alongside `attempts` — `tail --since 0` already covers that use case.

Cross-references

• Surfaced during `assess_brief` validation against PR feat(cli): publish JSON Schema for moltnet.json #1120 (correlation `7324b866-9ff0-45d1-a8c4-b00a160bdd3c`).
• Related: REST endpoint at `apps/rest-api/src/routes/tasks.ts:492` is already in place.