M1 — Task board MCP server

# Milestone M1 — Task board MCP server

You are building the first milestone of the **Agent Launchpad**: a platform where coding agents
connect over MCP, claim work, and submit it. M1 is the task board server agents talk to.

Build it in TypeScript under `src/`, as a **stdio MCP server** using `@modelcontextprotocol/sdk`
(listed in `package.json`, with its `package-lock.json`: `npm ci` installs it). It must start with `npm start` (which runs `tsx src/index.ts`).

## Configuration (environment variables)

| Variable | Required | Meaning |
| --- | --- | --- |
| `TASKS_FILE` | yes | Path to a JSON file describing the tasks (format below). Read-only for the server. |
| `STATE_FILE` | no | Path where the server persists mutable state. Default: `${TASKS_FILE}.state.json`. |
| `LEASE_SECONDS` | no | Lease duration in seconds. Default: `1800`. |

Tasks file format:

```json
{
  "tasks": [
    { "id": "T1", "title": "Scaffold repo", "priority": 1, "reward_shares": 100, "depends_on": [] },
    { "id": "T2", "title": "Add CI", "priority": 2, "reward_shares": 50, "depends_on": ["T1"] }
  ]
}
```

- `priority`: integer, 1 is the most urgent.
- `reward_shares`: positive integer.
- `depends_on`: ids of tasks that must be **submitted** before this task can be claimed.

## Task lifecycle

A task is `open`, `claimed` or `submitted`.

- **Claim** gives an agent a lease of `LEASE_SECONDS`. While the lease is active, only that agent can
  submit the task, and no other agent can claim it.
- When a lease expires, the task is `open` again (no claimer). Expiry is evaluated lazily at read or
  write time; no background timer is required.
- A task is **ready** when it is not submitted, has no active lease, and all its dependencies are submitted.
- An agent may hold **at most one active lease** at a time.
- Claiming a task you already hold with an active lease **renews** the lease (new expiry from now).
- Submitting is final: a submitted task can never be claimed or submitted again.

State (leases and submissions) must survive a server restart: persist it to `STATE_FILE` after every
change, and load it at startup.

## Tools

Every tool returns a single `text` content item whose text is JSON.

- On success: the JSON payload described below, `isError` absent or `false`.
- On failure: `isError: true` and the text is `{"error": {"code": "<CODE>", "message": "<human readable>"}}`.

Any invalid input (missing field, wrong type, empty string) must return the error code `INVALID_INPUT`
in this format, not a protocol-level error.

Timestamps are ISO 8601 strings in UTC (as produced by `Date.prototype.toISOString()`).

### `list_tasks`

Input: `{ "agent_id"?: string }`

Returns `{ "tasks": [TaskSummary, ...] }` with all **ready** tasks, plus, if `agent_id` is given, the
task currently leased by that agent (if any).

`TaskSummary` = `{ "id", "title", "priority", "reward_shares", "status", "claimed_by", "lease_expires_at" }`
(`claimed_by` and `lease_expires_at` are `null` when the task has no active lease).

Order: `priority` ascending, then `reward_shares` descending, then `id` ascending.

### `claim`

Input: `{ "task_id": string, "agent_id": string }`

Returns `{ "task_id", "agent_id", "lease_expires_at" }`.

Errors, checked in this order:

1. `TASK_NOT_FOUND` — unknown task id.
2. `ALREADY_SUBMITTED` — the task is submitted.
3. `ALREADY_CLAIMED` — another agent holds an active lease on it.
4. `NOT_READY` — at least one dependency is not submitted.
5. `LEASE_LIMIT` — this agent already holds an active lease on a different task.

### `submit`

Input: `{ "task_id": string, "agent_id": string, "commit_hash": string }`

`commit_hash` must be a full 40-character lowercase hexadecimal git SHA.

Returns `{ "task_id", "status": "submitted", "submitted_by", "commit_hash" }`. Submitting releases the lease.

Errors, checked in this order:

1. `INVALID_INPUT` — including a malformed `commit_hash`.
2. `TASK_NOT_FOUND`
3. `ALREADY_SUBMITTED`
4. `NOT_CLAIMED` — this agent has no active lease on the task (never claimed, claimed by someone else, or expired).

### `status`

Input: `{ "task_id": string }`

Returns `{ "id", "title", "status", "priority", "reward_shares", "depends_on", "blocked_by", "claimed_by", "lease_expires_at", "submitted_by", "commit_hash" }`.

- `blocked_by`: ids from `depends_on` that are not submitted yet, in `depends_on` order.
- Fields that do not apply are `null`.

Errors: `TASK_NOT_FOUND`.

## Constraints

- Do not modify anything under `tests/`. You may add your own tests elsewhere.
- Keep dependencies to what is installed unless you really need more.
- `npm test` runs the public tests. Hidden tests will also check the behaviour described above,
  including edge cases, concurrent calls on the same server, and persistence across restarts.