fix(sessions): dedupe cloud prompt echo when attachments add a summary

[tatoalo]

Problem

Creating a cloud task with an attachment rendered the user's prompt twice. The pinned optimistic bubble is seeded from the task description, which appends an Attached files: <names> line, while the echoed session/prompt carries attachments as resource_link blocks with no such line — so the text-based dedupe in mergeConversationItems never matched and both bubbles rendered.

Changes

• Add stripTrailingAttachmentSummary to @posthog/core/editor/cloud-prompt and fold it into the dedupe key in mergeConversationItems, so the description-seeded placeholder matches its echo.
• Carry the echoed prompt's attachments onto the pinned bubble when upgrading it, so the file chip replaces the literal summary line.
• Each pinned placeholder now consumes at most one matching echo, so a later follow-up repeating the same text still renders.
• Unit tests for the attachment, attachment-only, and repeated-text cases; verified end-to-end over CDP (fresh task with pasted image, real resume, cold reload).

[trunk-io]

😎 Merged manually by @tatoalo - details.

[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit 4dd4957.}

[greptile-apps]

_{Reviews (1): Last reviewed commit: "fix(sessions): dedupe cloud prompt echo ..." | Re-trigger Greptile}

[github-actions]

Targeted bug fix for cloud prompt deduplication — extracts a shared constant and stripping function, adjusts the merge logic to handle attachment summaries, and propagates echoed attachments onto the pinned bubble. Import direction is correct, all review comments were addressed, and test coverage follows conventions.

[greptile-apps]

_{Reviews (2): Last reviewed commit: "refactor(sessions): share attachment sum..." | Re-trigger Greptile}