v2.3.18

Fixed — bound MCP responses so heavy load can't crash the stdio pipe

Under heavy recall usage the MCP could disconnect mid-session (the whole Claude
Code session lost the tool). Root cause: search_messages returned each message's
full text verbatim via _row_to_hit; a single ~172k-char tool-result blob (times
limit rows) produced a JSON-RPC response large enough to break the stdio pipe.

• _row_to_hit caps bulky free-text fields (text/content/value/preview/
  body/snippet) at 1500 chars with <key>_truncated + <key>_full_chars markers
  — one chokepoint bounds every recall/message tool.
• search_messages clamps limit to ≤50 and applies a 256 KB total-response
  backstop that trims rows and appends a non-silent _meta.
• Internal Python callers bypass _row_to_hit, so they keep full text — no logic change.
• New tests/test_mcp_payload_bounds.py pins the bounds.