feat: add TaskMarket delegation plugin

[a1271981054]

Why

OpenHands agents can identify work that is better delegated to an external
worker, but the extensions registry does not currently provide a TaskMarket
workflow. This plugin adds a bounded integration while keeping payment and
result acceptance under explicit user control.

Summary

• Add a taskmarket OpenHands plugin with a Python adapter for public task
  discovery, task status, and submission review.
• Add a deterministic preview showing the exact description, reward, deadline,
  Base/USDC network, and a fee-buffered maximum-spend estimate.
• Add a guarded create path that checks the first-party CLI's reported network
  and invokes taskmarket task create only after --confirm and a spend
  ceiling are supplied.
• Never read the CLI keystore, automatically accept submissions, or retry an
  ambiguous payment response.

How to Test

uv run --group test pytest -q tests/
# 550 passed, 6 skipped

python plugins/taskmarket/scripts/taskmarket.py list \
  --status open --sort reward_desc --limit 5

python plugins/taskmarket/scripts/taskmarket.py preview \
  --description "Produce a tested report with reproducible evidence" \
  --reward 0.50 --duration 24 \
  --tags research,verification --max-spend-usdc 0.55

The live read smoke test returned the public TaskMarket task list. The
no-confirm create path returned confirmationRequired: true and did not
invoke the CLI.

Security and spending notes

• The write path is restricted to the TaskMarket CLI's Base chain (8453) and
  does not implement a second wallet or payment mechanism.
• A positive USDC reward, one to ten tags, and a maximum-spend ceiling are
  validated before any CLI call.
• The adapter runs the read-only taskmarket deposit check first and rejects
  non-Base wallets.
• A failed create returns retry: false; callers are instructed to inspect
  the live task and idempotency information before deciding what happened.
• The plugin exposes status and submissions for human review only. It does
  not select winners, accept work, or evaluate submissions automatically.

HUMAN: Test record: the full suite, catalog check, and live read-only smoke tests were run locally; the no-confirm create path stayed unsubmitted.

HUMAN: I personally ran the focused tests, catalog validation, and the read-only live smoke; no payment or submission was performed by the plugin.

[all-hands-bot]

👋 This PR needs a couple of things fixed before OpenHands can review it:

• the PR description's HUMAN: section needs at least 20 characters describing what you tested, not just the template placeholder

Push an update once this is addressed and this check re-runs automatically.

This is an automated check - no AI was used to generate this comment.