fix(urls): update Raycast MCP docs URL

[gavin-at-pieces]

• Raycast moved the MCP docs page under /ai/model-context-protocol.
• The old manual.raycast.com/model-context-protocol URL now returns 404.
• This updates RAYCAST_MCP_DOCS, which is used by pieces mcp docs --integration raycast.
• This is a one-line CI/user-facing link fix.
• No test behavior, workflow behavior, MCP handler logic, or readiness wording changes.

Validation:

• old URL returns 404
• new URL returns 200
• targeted RAYCAST_MCP_DOCS link test passed
• full tests/links_test.py passed: 56 passed, 1 skipped
• pieces mcp docs --integration raycast prints the new URL