feat: generate session titles locally, without sending an excerpt

[devGregA]

Problem

Session-title generation posted an excerpt of the conversation — the user's prompt and the assistant's reply — to the managed platform's /tools endpoint as chat_title, purely to produce a display string for the session list.

That is the only call in the product that sends conversation content anywhere other than the configured model provider. With a third-party model backend it is a second, unrelated destination for the same text, and the bundled web UI fires it unprompted on the first turn.

Upstream gated this behind an auto_session_title experimental flag. That flag no longer exists anywhere in the 2.0.0 tree — only the changelog mentions it — so there was nothing left to turn off.

Nothing is lost

applyPromptMetadataUpdate already sets a replaceable title locally from the first prompt via titleFromPromptMetadataText, and that text passes through the secret redactor first. Sessions stay titled. The title is a truncated prompt rather than a generated phrase.

What changed

Sealed in two places:

• fetchChatTitle is the chokepoint — the only function that sends chat_title — so refusing there also covers any caller a later upstream merge introduces. Upstream's body is kept as fetchChatTitleRemote, unreachable, so its tests keep running and upstream changes still merge cleanly.
• SessionTitleService.generateAndApply bails in front of it, so the disabled path does no provider lookup and never asks the OAuth token provider for an access token.

Two test-structure changes worth calling out, because they preserve coverage rather than delete it:

• composeTitleInput is now exported, so its per-prompt budget, first-turn truncation and digest-elision behaviour stay under test as a pure function rather than being asserted through a request body that is no longer sent.
• The SDK's serializes a temporary title-generation close against a public resume test keeps its real subject — a public resume queuing behind an in-flight close — by gating on the will-close hook instead of on an outbound request starting.

Verification

Typecheck, lint and build clean. The suite is back to the same pre-existing kap-server search failures as main.

All seven new tests were negative-controlled by restoring both call paths. Every one fails without the seals, at each layer:

• oauth: refuses without making a request, refuses for every caller
• engine: never calls the backend, whatever the source, does not reach for an OAuth token either, refuses even when forced, leaves the locally derived title in place, publishes no metadata event
• server: never sends a chat excerpt through the public REST title path (asserts the tools endpoint was never hit and the session keeps its local title)
• SDK: applies no generated title and sends no excerpt to the tools endpoint

🤖 Generated with Claude Code