Add NIP-C1: Agent TUI Messages

[lmanchu]

Summary

Defines a structured message format for AI agents to send interactive UI elements through Nostr DMs:

• text — structured text from agents
• buttons — tappable action buttons with callback protocol
• card — information cards with optional image and actions
• table — key-value data display

Complements NIP-AE (Agents) by standardizing the message layer between agents and clients. NIP-AE covers identity and discovery; NIP-C1 covers what agents actually say.

Design Decisions

• Embedded in DM content: TUI messages live inside NIP-17/NIP-04 encrypted message content as JSON. No new event kinds needed.
• Graceful degradation: Non-supporting clients see raw JSON. Spec includes plain-text fallback rendering rules.
• Callback protocol: Simple sigil:callback:<id> string for button responses. No new event kind, just a plain message back.
• Extensible: New type values can be added without breaking existing clients.

Reference Implementation

• sigil-core — Rust library with TuiMessage enum
• sigil-cli — Ratatui terminal rendering
• Sigil iOS — SwiftUI native component rendering
• Verified E2E with Damus interop (plain text fallback works)

Relationship to NIP-AE

NIP-AE defines agent definitions (kind 4199), lessons (kind 4129), nudges (kind 4201), and attribution. This NIP is orthogonal — it defines the message format agents use within existing DM infrastructure. An agent defined via NIP-AE would use NIP-C1 messages to communicate with users.