fix(side-chat): deliver reply anchor on first turn

[kongenpei]

What was wrong

Side-chat forks persisted the selected-message reply anchor as agent-only input, but deliberately started the native provider fork with empty input so it remained idle. The first real send built its provider command only from the new message, leaving the anchor out of the first provider turn. This affected Codex, Claude Code, and Pi before provider-specific translation. See #2316.

What changed

• Resolve deferred agent-only thread-start context before the first real provider-bound turn.
• Prepend that context to flat provider input and the first grouped input for direct and queued sends.
• Revalidate eligibility inside the send transaction so concurrent sends cannot deliver the anchor twice.
• Keep context deferred across a dispatch failure until a provider turn/started exists; later turns never receive it again.
• Exclude agent-only context from user-editable prompt history while retaining it on the accepted turn request.
• Always create reply context for a non-empty side-chat anchor, including the latest source message, and remove the unnecessary source-timeline lookup.
• Keep the initial native fork idle with an empty thread.start.input.
• Increment HOST_DAEMON_PROTOCOL_VERSION to 168 because turn.submit.input semantics change. The wire shape and database schema are unchanged.

How you verified

The deterministic first-turn assertion failed on main before the fix: expected [seed, firstReply], received only [firstReply].

• pnpm exec turbo run test --filter=@bb/server -- test/public/public-thread-fork.test.ts test/services/prompt-history.test.ts — 29 tests passed.
• pnpm exec turbo run test --filter=bb-plugin-side-chat — 25 tests passed.
• pnpm exec turbo run test --filter=@bb/host-daemon-contract -- test/contract.test.ts — 37 tests passed.
• pnpm exec turbo run typecheck --filter=@bb/server --filter=bb-plugin-side-chat --filter=@bb/db --filter=@bb/host-daemon-contract — 8 Turbo tasks passed.
• Browser flow with a side chat anchored to a BANANA instruction: before the fix the provider answered APPLE; after the fix it answered BANANA.

Fixes #2316

AGENT GENERATED