Fix MCP string result rendering

[cnYui]

Summary

• render FastMCP-style single-field { result: string } payloads as plain text instead of showing the JSON wrapper
• add a regression test for MCP tool result rendering
• isolate CLI smoke tests with a temporary KODE_CONFIG_DIR and add timeouts for two slow-prone tests

Verification

• bun test -> 552 pass, 8 skip, 0 fail across 144 files
• bun run typecheck
• bun run lint
• bun run format:check
• git diff --check
• git diff --cached --check