feat: selective message copying from session transcript #22648

Problem

Currently, when copying a session transcript, users can only copy the entire session or the last message. There is no way to select a subset of messages to copy. This is inconvenient when:

• You only need a specific exchange (e.g., one question and its answer) from a long session
• You want to share a portion of a conversation without irrelevant context
• You need to extract specific assistant responses without the surrounding back-and-forth

Proposed Solution

Add a new copyWithOptions command (slash command: /copy-with-options) that opens an interactive dialog where users can:

1. View a list of all session messages with previews (first 60 characters), model name, duration, and timestamps
2. Toggle individual messages for inclusion using Space
3. Select all messages with Ctrl+A
4. Navigate with arrow keys (with wrapping) and mouse hover/click
5. Confirm with Enter to copy the selected messages to the clipboard

The copied output should be a formatted Markdown transcript of only the selected messages, without the session metadata header (title, ID, timestamps), since a partial selection doesn't represent a full session export.

This command complements — not replaces — the existing copy command.

Acceptance Criteria

• New /copy-with-options slash command registered in the session command list
• Interactive dialog with keyboard-first navigation (up/down, space, ctrl+a, enter, esc)
• Mouse support (click to toggle, hover to highlight)
• Message previews show truncated text content, model/source info, and timing
• Scrollable list for sessions with many messages
• Warning shown if user tries to confirm with no messages selected
• Success toast with count of copied messages
• Error handling with user-facing toast on failure
• Existing copy and export commands unaffected
• Unit test coverage for selection logic and command handler workflow

Related

• May also help with selective export workflows