wip refactor: fold agent sessions on the backend only

[ehayes2000]

Rip out the wasm/browser half of the fold. The server already ran a fold machine per live session to keep placeholder rows in step; it is now also the only fold anywhere:

• GET /agent-sessions/channel/{id}/messages returns the folded messages (with the session bot, and answering empty rather than 404 for channels without a session). The raw-log endpoint is gone.
• The realtime path publishes the folded message each frame changed (agent_session_message: kind new/update + the whole message) instead of raw ACP frames. Frames that change nothing renderable publish nothing.
• Snapshot/stream alignment is a counter instead of content comparison: the snapshot carries logLength (frames folded), every event carries logIndex, and a reader drops events with logIndex <= logLength. This replaces the client-side buffered-frame overlap alignment entirely.
• agent_session gains a wire module: one serializable shape for folded messages, shared by the REST endpoint and the websocket event so the two cannot drift.

agent_fold loses its wasm inbound module, cdylib crate type, wasm dependencies and the cli feature split; agent_runtime_protocol loses its transport feature split; the hakari exclusions and workspace-hack memberships for the fold's dependency chain are restored.

The web app no longer folds: the worker, wasm module, hand-written types and the fold stream client are deleted. Channels fetch the folded snapshot and upsert websocket events into a reactive store keyed by agentSessionMessageId. Placeholder-row synthesis (and adoption by the real row) stays, as does hiding duplicate prompts.

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Preview: https://claude-acp-fold-backend-migrat-4jbjew.preview.macro.com/app (7cfaa23)