feat(doctor): add opt-in --live verification for opencli channels

[esaminu]

agent-reach doctor always shows Reddit, X, Facebook and Instagram as unverified, even when they work. Agents then skip them.

• I added agent-reach doctor --live, and the env var AGENT_REACH_DOCTOR_LIVE=1 does the same.
• It runs one read-only opencli command per channel and marks the channel ok when the command works.
• Passes are cached for 6 hours and failures for 15 minutes, so the check does not hit rate limits.
• Without the flag, doctor output does not change.