feat: ask the harness to title a session

[dviejokfs]

What

Lets a caller ask the harness to title a new session, the way the harness's own client does.

• TurnRequest::request_session_title: the user's own description of the task (normally the first prompt, without injected context). Keep setting it until a title arrives.
• TurnEvent::SessionTitle and TurnResult::session_title: the title, trimmed and de-duplicated.
• Carried over the protocol (ProtocolTurnInput::request_session_title) and never logged.

Per harness:

• Claude Code: generate_session_title, asked after the turn's first frame.
• Codex app-server: replays the TUI's hidden, ephemeral, read-only title thread on the turn's own connection, with every tool disabled and one fallback model attempt.
• OpenCode serve: titles through its session API.

Generation runs alongside the turn and never delays or fails it. A turn that ends first simply reports no title.

Why

Temps Fleet's uncommitted src/agents/runtime_sdk.rs change uses SdkTurnEvent::SessionTitle and request_session_title. Fleet's pin (dafba07) doesn't have them, so Fleet can't commit that change until this merges and the pin moves.

Verification

cargo fmt --check, cargo clippy --locked --all-targets --all-features -- -D warnings and cargo test --all-features --lib (442 passed) are clean locally. CI runs a newer clippy than my local toolchain, so its first run may report lints I could not reproduce.

🤖 Generated with Claude Code

[greptile-apps]

Confidence Score: 5/5

[Medium impact] The PR appears safe to merge; all previous findings are addressed.

Summary

This PR lets callers ask the harness for a session title and receive it through SessionTitle.

• Callers can ask supported harnesses to title new sessions.
• Codex titles new sessions on a separate hidden thread.

Diagram

%%{init: {'theme': 'neutral'}}%%
flowchart TD
  A[Caller requests a title] --> B[User turn starts]
  B --> C[Harness title work runs alongside turn]
  C --> D{Title arrives first?}
  D -->|Yes| E[Report SessionTitle]
  D -->|No| F[Finish turn without a title]
  C --> G[Offer optional provider writes]
  G --> H{Writer accepts bytes?}
  H -->|None| I[Drop optional frame]
  H -->|Some| J[Keep unfinished bytes]
  J --> K[Send remainder before later frames]

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_{Reviews (5) · Last reviewed commit: "fix(runtime): keep the unsent rest of a ..." · Reviewed by Greptile}