Add MCP proof lookup tool

[newmattock]

Summary:
Adds a read-only get_proof MCP tool for looking up public proof records by hash.

Related bounty or issue:
Bounty #37

What changed:

• Added get_proof to the MCP tools/list response.
• Implemented tools/call handling for get_proof using existing Proof rows and stored public proof JSON.
• Returned useful wrapper metadata (hash, kind, ledger/submission ids, creation time) without changing proof payloads, ledger hashes, or payout behavior.
• Added a short get_proof MCP example to docs/agents.md.
• Added tests for a found proof and an unknown proof hash.

Validation:

• ../mergework-bounty-20/.venv/bin/python -m pytest tests/test_api_mcp.py::test_mcp_get_proof_returns_public_proof_details tests/test_api_mcp.py::test_mcp_get_proof_reports_unknown_hash -q -> 2 passed.
• ../mergework-bounty-20/.venv/bin/python -m pytest tests/test_api_mcp.py -q -> 13 passed.
• ../mergework-bounty-20/.venv/bin/python -m pytest -q -> 74 passed.
• ../mergework-bounty-20/.venv/bin/python -m ruff format --check . -> 28 files already formatted.
• ../mergework-bounty-20/.venv/bin/python -m ruff check . -> All checks passed.
• ../mergework-bounty-20/.venv/bin/python -m mypy app -> Success.
• ../mergework-bounty-20/.venv/bin/python scripts/check_agents.py -> AGENTS.md ok.
• ../mergework-bounty-20/.venv/bin/python scripts/docs_smoke.py -> docs smoke ok.

Notes:
Documentation and MCP behavior only; no deployment, secrets, ledger mutation, proof payload mutation, or payout behavior changes.

[Zejia]

No blockers from my pass.

Checked:

• app/main.py adds get_proof to tools/list and returns wrapper metadata plus the stored public proof payload. It does not change proof generation, ledger hashes, or payout behavior.
• tests/test_api_mcp.py covers both an existing proof hash and an unknown hash.
• docs/agents.md adds a short MCP example and the tool list entry.

Validation on this PR branch:

• python -m pytest tests/test_api_mcp.py::test_mcp_get_proof_returns_public_proof_details tests/test_api_mcp.py::test_mcp_get_proof_reports_unknown_hash -q -> 2 passed.
• python -m pytest tests/test_api_mcp.py -q -> 13 passed.
• python -m ruff format --check . -> 28 files already formatted.
• python -m ruff check . -> All checks passed.
• python -m mypy app -> Success.
• python scripts/check_agents.py -> AGENTS.md ok.
• python scripts/docs_smoke.py -> docs smoke ok.