[FEATURE]: selective message copying from session transcript #22871

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

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

• Resubmission of feat: selective message copying from session transcript #22648, which was closed for not following the feature request template (missing [FEATURE]: prefix, verification checkbox, and template structure). This issue corrects those compliance issues.
• feat: Copy Mode for OpenCode #2755 ("feat: Copy Mode for OpenCode") was flagged as a potential duplicate by the compliance bot. While both concern copy functionality, this request is specifically about selective message copying via an interactive picker dialog with the scope and acceptance criteria above — distinct from the broader "copy mode" concept in feat: Copy Mode for OpenCode #2755.
• May also help with selective export workflows.