fix(mcp): expose structured results to text-only clients

[davidrobertson]

Summary

• serialize each structured MCP result into the first text content block for text-only clients
• preserve the short human summary and the existing structuredContent payload
• update the compiled package output and add regression coverage

Validation

• npm run typecheck
• npm test (413 passed, 2 Windows-only skips)
• npm run smoke
• npm pack --dry-run
• MCP integration suite (34 passed)
• synthetic Cursor capture followed by real stdio lcm_grep and lcm_pack_context; serialized text parsed equal to structuredContent
• independent QA, code review, and final gate approved commit 81133f74ff77f9c487054f335d8fc24767aad187

Risk

Structured results now appear in both text and structured form. This increases response bytes for large results, but existing search, paging, and packing limits still bound the payloads.