fix(sessions): keep auto task titles pinned to the original prompt

[k11kirky]

Problem

Tasks sometimes get renamed based on the most recent messages in the conversation instead of keeping the name generated from the original prompt.

The only writer that rewrites a task title after creation is useChatTitleGenerator: its interval and catch-up fires (REGENERATE_INTERVAL) generate from the last 7 prompts and write the result as the task title whenever the title isn't locked (title_manually_set). Tasks created server-side (cloud creation, warm-run activation, Slack-origin tasks) get an auto-generated title with title_manually_set = false, so they are exactly the ones exposed. #3315 removed the remount-driven refires, but any surviving prompt-window fire still renames these tasks. Task description is never rewritten anywhere; the rename is the whole symptom.

Changes

• Add canApplyTitleFromPrompts in chatTitle.ts: prompt-based fires past the first prompt may no longer write the title, unless the current title is still the raw-description placeholder (so the retry path for failed initial generations keeps working).
• Gate the title write in useChatTitleGenerator with it; summary generation is unchanged, so conversation summaries (used for commit/PR context) still refresh at every interval.

How did you test this?

Red-green: added a failing test reproducing the rename (unlocked task with a real title at prompt count 8 gets retitled from recent prompts), then implemented the gate and confirmed it passes. Also added coverage for the placeholder-retry path. Ran the full sessions suites in @posthog/core (274 tests) and @posthog/ui (365 tests), biome lint, and tsc --noEmit on both packages — all green.

Automatic notifications

• Publish to changelog?
• Alert Sales and Marketing teams?

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[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit b8bd2c0.}

[tatoalo]

thanks!

[greptile-apps]

_{Reviews (1): Last reviewed commit: "fix(sessions): keep auto task titles pin..." | Re-trigger Greptile}