docs(chatkit): document identities, delegation scope, and external replies

[danielblignaut]

Companion to trytilde/api#172.

A ChatKit session can now hold several people and several agents at once, including people who arrived through GitHub or Slack and have no Tilde account. Documents:

• ChatKit identity — what addresses each participant, and why the same handle in two GitHub organizations stays two people.
• The invocation rule — an agent takes a turn only when a message addresses it; everything else arrives as context.
• Unlinked external identities carry no permissions. An address on its own proves nothing about who sent it. Stated in both the human page and llms/chatkit.md because it is the rule most likely to be worked around by an agent trying to be helpful.
• External replies — Tilde now delivers the agent's reply back to the platform the conversation started on, so an agent must not also post it with a provider tool, or the recipient gets it twice.
• Session-scoped MCP connections — x-tilde-chatkit-session-id or chatkit: { sessionId }, and the fact that a session the caller may not use rejects the connection rather than silently dropping the scoped tools.

Human pages carry the concepts; llms/chatkit.md carries the exact headers, field names, and rules an agent needs.

Validation

mint validate passes. git diff --check clean. No tool-catalog change, so no provider regeneration is required.

🤖 Generated with Claude Code

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