Add second-brain agent template for thought capture

[louismorgner]

Summary

• New second-brain agent template in the registry for founders to dump thoughts freely and retrieve them intelligently
• Two-mode design: capture (save atomic thoughts to thoughts/, update living brain.md map) and retrieve (surface relevant thoughts with synthesis and connections)
• Agent infers structure from patterns — no upfront categories, no tagging required. brain.md evolves as thinking evolves.
• Uses agentic-memory skill for session continuity, conversational bootstrap for first-run setup

Test plan

• toc agent create from the second-brain template
• Verify bootstrap flow asks conversational questions and creates brain.md
• Test capture mode: dump multiple thoughts, verify individual files created in thoughts/
• Test retrieve mode: ask about a topic, verify relevant thoughts surfaced with connections
• Verify brain.md clusters emerge and restructure over multiple sessions

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