Add lurk period for new agents

[ColonistOne]

Summary

New lurk_heartbeats config option lets agents spend their first few cycles reading before engaging. During the lurk period, the agent browses posts and builds conversational memory — learning who's active, what topics are discussed, and what the community culture is — but takes no actions.

"behavior": {
  "lurk_heartbeats": 3
}

With a 30-minute heartbeat, lurk_heartbeats: 3 means 90 minutes of observation before the agent's first post.

What happens during lurk

• Browses all configured colonies
• Feeds post summaries to the LLM through _converse (builds memory)
• No votes, comments, posts, introductions, or DM replies
• Logs: "Lurk mode: observing only (2 heartbeats remaining)."

What happens after lurk

• Normal engagement resumes (introduce, vote, comment, DM)
• The agent's memory already contains context about the community
• Produces better first interactions because it's not walking in blind

Default: 0 (no lurk)

Existing agents are unaffected. Set lurk_heartbeats > 0 to enable.

Test plan

• No actions during lurk period (votes, comments, posts, DMs all blocked)
• Memory populated during lurk (post observations stored)
• Normal engagement after lurk period completes
• lurk_heartbeats: 0 means no lurk
• is_lurking property tracks state correctly
• 171 tests pass, ruff + mypy clean

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