[FEATURE]: Expose messages.copy through the slash-command surface

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

The TUI currently has two copy actions:

• messages.copy copies the last assistant message and is available through <leader>y / the command palette.
• session.copy copies the full session transcript and owns /copy.

I found this a little surprising. I first tried /copy expecting the latest assistant response, and only later found Copy last assistant message in the palette.

It also seems inconsistent with several other TUI actions, where the keybind and slash-command surfaces map to the same operation:

/compact  <-> Ctrl+X C
/editor   <-> Ctrl+X E
/export   <-> Ctrl+X X
/models   <-> Ctrl+X M
/new      <-> Ctrl+X N
/undo     <-> Ctrl+X U

???       <-> Ctrl+X Y  (Copy last assistant message)
/copy     <->           (Copy session transcript)

Was keeping /copy mapped to the full transcript while messages.copy has no slash command an intentional UX decision?

There are a few possible directions:

• expose the existing action as /copy-message or /copy-last
• make /copy copy the latest response and move the transcript action to /copy-session
• use a picker / indexed form for older responses

Related approaches have already been proposed in #30548 (message picker) and #34044 (/copy N). This issue is narrower: I'd first like to clarify what the intended slash-command mapping for the already-existing messages.copy action should be.

Related: #26240, #27967, #30562.

I'm happy to implement whichever direction the team prefers once the command semantics are agreed on.