Add manual session deletion UI with confirmation dialog

Feature Request

Problem

Currently, there is no way to manually delete individual conversation sessions from the Claude Code extension UI (VS Code, Antigravity, etc.). Users must either:

• Wait for automatic cleanup based on cleanupPeriodDays setting
• Manually navigate to ~/.claude/projects/ and delete session files

This creates a disappointing user experience when users want to clean up their conversation history immediately or selectively.

Proposed Solution

Add manual session deletion capability to the Past Conversations dropdown in the extension with the following features:

Option 1: Inline Delete Button

• Add a small 'x' button or trash icon at the end of each row in the Past Conversations dropdown
• Clicking it triggers a confirmation dialog before deletion

Option 2: Enhanced Session Manager

• Create a modal/popup (similar to Antigravity's UI patterns) that displays all past sessions in a scrollable list
• Add a trash can icon at the end of each row for deletion
• Include session metadata (date, message count, etc.) to help users identify sessions

Important UX Considerations

Confirmation Dialog (Critical)

• MUST include a confirmation dialog/modal before deleting any session
• Prevents accidental deletions
• Dialog should clearly indicate:
• Which session is being deleted
• That this action is irreversible
• Require explicit user confirmation (e.g., "Delete" button, not just "OK")

Additional Features (Nice to Have)

• Bulk selection for deleting multiple sessions at once
• Search/filter functionality for finding specific sessions
• Option to "Delete all sessions" with appropriate warnings

Current Workaround

Users must manually delete project directories from ~/.claude/projects/[project-name]/ or configure cleanupPeriodDays: 0 to delete all sessions at startup.

Context

This feature request is specifically for the extension UI (VS Code extension, Antigravity, etc.), not the CLI interface.

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This feature would significantly improve the user experience and give users more control over their conversation history management.