v0.1.1 — receipts as MCP tools

Adds the SignalBrain MCP server: pip install "signalbrain[mcp]" → sb-mcp (stdio). Tools: emit_receipt (spec-compliant, self-validating, refuses guessed dates and disallowed measure commands), validate_receipt, gate_status. Works with any MCP client — goose, Claude Desktop, Cursor. Live on PyPI. Note: the tokenless release workflow's publish job will show red until PyPI trusted publishing is configured (one-time browser form); this release was published directly.