fix(runtime): isolate Fitbit candidate port

[kachofugetsu09]

Summary

• declare the Fitbit monitor exclusive endpoint for candidate isolation
• make both monitor and MCP honor FITBIT_MONITOR_PORT
• expose only four read-only Fitbit tools to candidate validation
• publish 1.4.2 for a fresh Core-owned plugin venv

Incident evidence

• 1.4.1 candidate correctly aborted because the changed managed service lacked validation_port_env
• the stable 1.4.0 endpoint remained running throughout

Verification

• targeted plugin and runtime-env tests: 10 passed
• full local suite collection is unavailable because the desktop Python environment lacks the declared mcp dependency; repository CI remains authoritative
• git diff --check