feat(acp): stream session/prompt deltas as session/update chunks (#3702)

Refs #3192.

Streams ACP session/prompt text deltas as session/update chunks instead of buffering the full provider response, while preserving the single-writer stdout path and mid-stream session/cancel behavior from #3698.

Verification: GitHub CI green across lint, Ubuntu/macOS/Windows tests, npm/mobile smoke, DCO/gate, Version drift, Claude review, and GitGuardian. Local targeted verification also passed in an isolated worktree: cargo test -p codewhale-tui --bin codewhale-tui --locked acp_server:: -- --nocapture (18 passed).

Thank you @findshan for the careful ACP streaming slice and @Jengro777 for keeping the ACP registry/Zed path concrete.

Author: Hmbown