Revert "rotating empty state messaging"

[justschen]

Reverts #327542

[Copilot]

Pull request overview

Reverts the rotating chat placeholder feature, restoring static placeholder behavior.

Changes:

• Removes placeholder rotation and shimmer transitions.
• Restores one-time randomized placeholders for new sessions.
• Updates Sessions documentation accordingly.

Show a summary per file

File	Description
chatInputPlaceholderRotation.ts	Removes rotation utilities and shared messages.
chatWidget.ts	Restores static agent placeholders.
SESSIONS.md	Removes rotation documentation.
newChatInput.ts	Restores per-widget random placeholder selection.
placeholderTextContribution.ts	Removes transition support.
placeholderText.css	Removes shimmer animation styles.

Review details

• Files reviewed: 6/6 changed files
• Comments generated: 0
• Review effort level: Medium

[justschen]

@copilot resolve the merge conflicts in this pull request

[Copilot]

@copilot resolve the merge conflicts in this pull request

Merge conflicts resolved in 28909ff.

The only conflict was chatInputPlaceholderRotation.ts (deleted by this revert, modified on main by #327684 to skip rotation while dictating). Since the whole rotation feature is being reverted, I kept the deletion. activeDictationEditor() is still used by editorDictation.ts, so nothing on the dictation side breaks, and no references to the removed module remain. npm run typecheck-client passes.

[vs-code-engineering]

This PR will be automatically cherry-picked to release/1.131 when merged.