fix(tui): honor the auto_session_title experimental flag

[iosmanthus]

Related Issue

Related to #2963 — this covers the AI-session-title part of that report. The two other
asks in it (hiding the third-party-provider OAuth error toast, and picking the node used
for titles) are out of scope here.

Note on process: #2963 has no maintainer /approve yet. Happy to wait for one, or to
split this into a separate bug issue first if you would rather have it filed that way —
the patch is small and can sit here in the meantime.

Problem

auto_session_title generation is on demand only: the engine never starts one by itself,
a client has to ask. kimi web asks every time a turn settles, with
generateSessionTitle(sessionId, { source: 'first_turn' }) and a 3-attempt-per-session
cap. The TUI never asks — there is no generateTitle / generateSessionTitle call site
anywhere under apps/kimi-code, only the kap-server route and the klient facade.

The TUI does list the flag in /experiments, description included:

 ❯ AI session titles  disabled
   id auto_session_title · default · KIMI_CODE_EXPERIMENTAL_AUTO_SESSION_TITLE
   Generate concise session titles from the conversation through the managed chat_title
   tool: clients auto-generate once the first turn completes and offer on-demand
   regeneration in the rename field.

So a CLI user reads "clients auto-generate once the first turn completes", turns the
switch on, and nothing happens. The session keeps the titleKind: 'replaceable' title
that applyPromptMetadataUpdate derived from the first prompt, which the terminal title
then truncates to 32 characters.

What changed

KimiTUI.handleTurnEnded now asks the engine for a title, with the same policy the web
client uses:

• only after a turn that ended completed, so the first_turn excerpt has both a user
  prompt and an assistant reply (the web client asks on any settle; restricting it to
  completed turns keeps a cancelled first turn from naming the session)
• source: 'first_turn'
• at most 3 attempts per session, and none once a title lands
• fire and forget: a rejected request leaves the prompt-derived title in place

Nothing downstream needed changing. The engine already refuses to overwrite a custom or
generated title, skips the platform call entirely without a managed OAuth login, and
publishes session.meta.updated, which handleSessionMetaChanged already turns into the
status-panel title and the terminal title.

Also adds the missing KIMI_CODE_EXPERIMENTAL_AUTO_SESSION_TITLE row to the env-var
reference (en + zh), which listed the other two KIMI_CODE_EXPERIMENTAL_* flags but not
this one.

Verification

• 4 new tests in test/tui/kimi-tui-message-flow.test.ts: asks once on a completed turn
  and not again after a title lands, stays quiet while the flag is off, ignores a
  cancelled turn, gives up after 3 unproductive attempts. Reverting the kimi-tui.ts
  change fails 2 of them.
• apps/kimi-code suite: 3136 passed, 2 skipped, no regressions.
• tsc -p apps/kimi-code/tsconfig.json --noEmit clean; oxlint reports no new findings.
• End to end against a real managed login: with
  KIMI_CODE_EXPERIMENTAL_AUTO_SESSION_TITLE=1, a fresh session's terminal title changed
  from the truncated prompt to TypeScript readonly 数组与普通数组区别说明, and its
  state.json went from titleKind: 'replaceable' to 'generated'.

Not included, happy to add if you want it in the same PR: the "on-demand regeneration in
the rename field" half of the flag description — a /title --regenerate (or bare /title
on an already-titled session) that calls generateSessionTitle({ force: true }).

Checklist

• I have read the CONTRIBUTING document.
• I have linked a related issue (external PRs: the issue must have a maintainer's /approve).
• I have added tests that prove my feature works.
• Ran gen-changesets skill, or this PR needs no changeset.
• Ran gen-docs skill, or this PR needs no doc update.

[changeset-bot]

🦋 Changeset detected

Latest commit: 501841d

The changes in this PR will be included in the next version bump.

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@moonshot-ai/kimi-code	Patch

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