[BUG] Auto-generated session title overwrites the Fork: marker on forked sessions, producing duplicate titles

Problem

After forking a session, the new session is titled Fork: <source title>. Once the first message is sent in the forked session and auto title generation runs, the generated title replaces the whole title — the Fork: marker is lost. Since a forked session starts from the same context as its source, the generated title tends to be identical or very similar to the source session's title, leaving two same-named sessions in the session list that cannot be told apart.

Steps to reproduce

1. Open a session that already has a title (e.g. Fix login bug).
2. Fork it via /fork or kimi --resume --fork; the new session shows as Fork: Fix login bug.
3. Send a message in the forked session and wait for auto title generation to complete.

Expected

The forked session keeps its fork marker (keeps the Fork: prefix, or the generated title still carries a fork indicator), so it stays distinguishable from the source session in the list.

Actual

The auto-generated title replaces the whole title and drops the Fork: prefix; both sessions show the same title.

Root cause

• packages/agent-core-v2/src/workspace/sessionLifecycle/sessionLifecycleService.ts: on fork, the title is set to Fork: ${sourceTitle} and, when no explicit title is passed, titleKind is recorded as 'replaceable'.
• packages/agent-core-v2/src/session/sessionTitle/sessionTitleService.ts, generateTitleOnce(): generation is skipped only for titleKind === 'custom' or 'generated'; 'replaceable' titles — including the Fork: one — proceed to generation.
• packages/agent-core-v2/src/session/sessionMetadata/sessionMetadataService.ts, setGeneratedTitleIfUncustomized(): likewise only guards 'custom', so the Fork: title is overwritten outright.
• Nothing in the flow checks meta.forkedFrom or preserves the Fork: prefix.

Suggested fix (options)

• Skip auto title generation when meta.forkedFrom !== undefined; or
• When applying a generated title to a session with forkedFrom, keep the Fork: prefix; or
• Give the forked title its own titleKind that is not auto-replaced unless forced.

Note: whether force: true (manual regeneration) should overwrite the Fork: marker is a product decision and can be defined separately — this issue only concerns non-forced automatic generation.

Start in packages/agent-core-v2/src/workspace/sessionLifecycle/sessionLifecycleService.ts to trace how forked titles and titleKind are set. Then read generateTitleOnce() in sessionTitleService.ts and setGeneratedTitleIfUncustomized() in sessionMetadataService.ts. Confirm that non-forced automatic generation preserves the fork distinction, and add or update coverage for the fork-and-generate flow if tests are found nearby.