fix(mcp): align delete receipts and semantics with REST

[jiashuoz]

Summary

Align MCP delete tools with the REST contract:

• return the typed API-key deletion receipt instead of fabricating { deleted: id }
• describe agent deletion as a 30-day soft delete, not a permanent cascade
• describe domain deletion as non-cascading and surface domain_has_agents
• lock all five existing MCP delete tools to their typed receipt shapes

Client surface checklist

• MCP tool behavior and descriptions
• Exact typed deletion-receipt tests for all existing delete tools
• No REST/OpenAPI/generated SDK changes required

Operational risk

Low. This changes MCP response fidelity and tool descriptions only; REST behavior is unchanged.

Test plan

• Focused MCP tests: 99 passed
• Full MCP suite: 178 passed
• MCP TypeScript build
• git diff --check