Implement open prompt in system text editor via Ctrl+X

[Copilot]

Adds the ability to open the current prompt in a system text editor during interactive mode, similar to Claude Code (Ctrl-G) and Gemini CLI (Ctrl-X).

Changes

• New keyboard shortcut: Ctrl+X opens the current buffer in an external editor
• Editor selection: Uses $VISUAL → $EDITOR → vi (Unix) / notepad (Windows)
• Terminal handling: Properly switches from raw mode to normal mode for the editor, then restores raw mode after
• Help docs: Updated /? shortcuts to document the new binding

Implementation

• cmd/cli/readline/editor.go / editor_windows.go: Platform-specific editor invocation
• cmd/cli/readline/types.go: Added CharCtrlX = 24
• cmd/cli/readline/readline.go: Handler for CharCtrlX that opens editor and replaces buffer with result

Usage

> type some text here, then press Ctrl+X
# Editor opens with "type some text here"
# Save and close editor
# Buffer now contains edited content

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• cloud.google.com
  • Triggering command: /update-job-proxy /update-job-proxy (dns block)
  • Triggering command: /update-job-proxy /update-job-proxy x64/pkg/tool/linux_amd64/compile (dns block)
• dario.cat
  • Triggering command: /update-job-proxy /update-job-proxy (dns block)
  • Triggering command: /update-job-proxy /update-job-proxy x64/pkg/tool/linux_amd64/compile (dns block)
• go.googlesource.com
  • Triggering command: /update-job-proxy /update-job-proxy (dns block)
  • Triggering command: /update-job-proxy /update-job-proxy x64/pkg/tool/linux_amd64/compile (dns block)
• go.opentelemetry.io
  • Triggering command: /update-job-proxy /update-job-proxy (dns block)
  • Triggering command: /update-job-proxy /update-job-proxy x64/pkg/tool/linux_amd64/compile (dns block)
• go.uber.org
  • Triggering command: /update-job-proxy /update-job-proxy (dns block)
  • Triggering command: /update-job-proxy /update-job-proxy x64/pkg/tool/linux_amd64/compile (dns block)
• go.yaml.in
  • Triggering command: /update-job-proxy /update-job-proxy (dns block)
  • Triggering command: /update-job-proxy /update-job-proxy x64/pkg/tool/linux_amd64/compile (dns block)
• gonum.org
  • Triggering command: /update-job-proxy /update-job-proxy (dns block)
  • Triggering command: /update-job-proxy /update-job-proxy x64/pkg/tool/linux_amd64/compile (dns block)
• google.golang.org
  • Triggering command: /update-job-proxy /update-job-proxy (dns block)
  • Triggering command: /update-job-proxy /update-job-proxy x64/pkg/tool/linux_amd64/compile (dns block)
• gopkg.in
  • Triggering command: /update-job-proxy /update-job-proxy (dns block)
  • Triggering command: /update-job-proxy /update-job-proxy x64/pkg/tool/linux_amd64/compile (dns block)
• gotest.tools
  • Triggering command: /update-job-proxy /update-job-proxy (dns block)
  • Triggering command: /update-job-proxy /update-job-proxy x64/pkg/tool/linux_amd64/compile (dns block)
• howett.net
  • Triggering command: /update-job-proxy /update-job-proxy (dns block)
  • Triggering command: /update-job-proxy /update-job-proxy x64/pkg/tool/linux_amd64/compile (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

Original prompt

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This section details on the original issue you should resolve

<issue_title>Implement open prompt in system text editor</issue_title>
<issue_description>As seen in claude code via ctrl-g and gemini-cli via ctrl-x</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Implement open prompt in system text editor docker/model-runner#468

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