[BUG] Built-in local slash command output (e.g. /context) doesn't create a rewind checkpoint, while plugin commands do

Bug Description
Rewind checkpoint behavior is inconsistent across slash commands.

Plugin-provided slash commands (e.g. /plugin …) DO create rewind-eligible
points — I can rewind to/past them and drop their output.

But built-in local commands such as /context do NOT. Their output is added to
the conversation and counts toward the context window, yet the Rewind picker
shows no checkpoint for them. There's no way to rewind to "before /context",
so the output stays in context and can't be removed via rewind.

Expected
Built-in local command invocations should be rewind-eligible (consistent with
plugin commands), so users can roll back and drop their output.

Actual
Cannot rewind to/past a built-in local command's output (e.g. /context),
even though equivalent plugin commands are rewindable.

Repro

1. Run /context (or any built-in local command).
2. Open Rewind (/rewind or Esc Esc).
3. No checkpoint exists for the /context step — only user-typed messages.
4. Compare: running a plugin slash command produces a rewindable point.

Environment

• Version: 2.1.158 (Claude Code)
• Platform: darwin (macOS 26.5)
• Terminal: xterm-ghostty