Add clipboard copy target for run output

[jlowin]

Add the ability to copy Colin's run output directly to the clipboard, formatted as XML to clearly delineate each output document.

Use case

When using Colin to generate context for AI assistants (e.g., pasting into Claude), it would be convenient to run a model and immediately paste the output without needing to pipe through additional tools.

Proposed interface

colin run --copy

This would:

1. Render all outputs as usual
2. Format them as XML with clear document boundaries
3. Copy the result to the system clipboard

Example output format

<documents>
  <document path="context/overview.md">
    ... rendered content ...
  </document>
  <document path="context/api.md">
    ... rendered content ...
  </document>
</documents>

Alternatives considered

• Piping to pbcopy/xclip manually works but adds friction
• A dedicated copy target in colin.toml could also work, though the CLI flag is more ergonomic for ad-hoc use