feat: add Taskmarket action provider

[MohDev212]

What problem this solves

Agents built on AgentKit currently have no first-party way to find or fund work on Taskmarket (https://taskmarket.dev), an onchain agent task marketplace that pays USDC on Base via the x402 protocol. This provider lets an AgentKit agent browse open tasks and create funded tasks from inside its own toolkit, with explicit user authorization gates.

Change summary

• Adds a TaskmarketActionProvider to the Python AgentKit package with three actions:
  • taskmarket_browse_tasks (read-only): lists open tasks from the public Taskmarket REST API with optional filters (max/min reward in USDC, mode, limit).
  • taskmarket_get_task (read-only): fetches task detail (status, reward, expiry, submission count, description) by task ID.
  • taskmarket_create_task (write): creates a task through the first-party taskmarket CLI via subprocess, so the CLI owns the wallet, x402 payment flow, and EIP-191 signatures. No API keys or private keys are stored or handled by this provider.
• Registers the provider in action_providers/__init__.py and the top-level package __init__.py.
• Adds changelog fragment and README documenting setup and safety gates.

Safety gates

• create_task requires an explicit confirmation boolean; without it the action refuses.
• Enforces a spending limit: TASKMARKET_MAX_SPEND_USDC env var (default 10 USDC); rewards above the limit are refused.
• Echoes the exact description, reward, duration, and Base network before executing.
• Never retries a payment whose settlement status is unknown (CLI timeout returns an error; no blind retry).
• No secrets are ever logged or committed.

Test plan

• 26 unit tests pass, all in tests/action_providers/taskmarket/:
  .venv/Scripts/python -m pytest tests/action_providers/taskmarket/ -q -> 26 passed
• Tests mock subprocess and HTTP; no live API calls, no wallet/keystore touched.
• Verified zero existing taskmarket support in this repo (git grep -i taskmarket).

[cb-heimdall]

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