The MCP server and the settle verb: sdcreceipt now covers the whole exchange
rather than everything after the hard part.

- `sdcreceipt-mcp`, an MCP stdio server exposing verify_receipt, sign_trigger
  and settle. Settlement is now the third verb an agent can drive, after
  sdcvalidator and sdcgovernance.
- `sdcreceipt settle`, which asks an issuer for a Receipt and prompts for the
  fields nobody can guess: the release condition is hashed and never stored,
  and the workflow states live in a schema the issuer holds.
- `examples/settle.py`, one settlement end to end offline on a bare install.

The MCP posture, which is the part to read before extending it: no tool takes
a URL, the signing key and the issuer endpoint are fixed at start-up, key
generation is not exposed, and submission is not exposed. Enforced by tests
and by CI, not by documentation.