fix: bound MCP live message responses

[vycdev2]

Summary

• Bound read-messages MCP responses to a configurable 120,000-character default, retaining the newest live messages when a response is oversized.
• Preserve the existing JSON response and image hint for responses within the configured limit.
• Document MCP_READ_MESSAGES_MAX_CHARS and add behavioral coverage through the public MCP tool path.

TDD evidence

• RED on untouched upstream/main: node --test tests/mcpReadMessagesOutput.test.mjs failed because the 100-message live MCP response exceeded 120,000 characters.
• GREEN after the fix: the same focused test passed, retaining newest-marker, omitting oldest-marker, and reporting truncation.

Verification

• npm ci — passed; npm reported 13 audit vulnerabilities and a pending node-pty@1.1.0 install-script approval warning.
• npm run build — passed.
• node --test tests/mcpReadMessagesOutput.test.mjs — passed (1 test).
• npm test — passed (66 tests).
• node --input-type=module -e 'const p=await import("node-pty"); console.log(typeof p.spawn)' — passed (function).
• git diff --cached --check — passed before commit.
• npm run format:check — unavailable; the repository defines no format script.
• No linter is configured in the repository.

Risk

Low. Normal-sized read-messages responses are byte-for-byte unchanged. Oversized responses now retain the newest complete message entries and append an explicit truncation note; no Discord API request behavior or message fields changed.

This was generated by an AI agent (vycdev2). Please verify any changes before merging or applying.