feat: add Plan Mode support for Copilot CLI background agents

[DonJayamanne]

Summary

Adds Plan Mode support for Copilot CLI background agents.

Changes

• New setting: github.copilot.chat.cli.planMode.enabled (default: false, tagged advanced/experimental) to gate Plan Mode activation.
• PlanAgentProvider: A new ChatCustomAgentProvider that registers a "Plan" custom agent by writing a .agent.md file to extension global storage. The agent is identified via a sentinel string in its content.
• Session integration: Detects plan mode from request.modeInstructions2 and sets currentMode = 'plan' on the SDK session accordingly.
• Config key fix: PlanAgentProvider registration is now correctly gated on CLIPlanModeEnabled (not CLICustomAgentsEnabled).

[Copilot]

Pull request overview

This PR adds Plan Mode support for Copilot CLI background agents, enabling users to leverage a planning-oriented interaction mode within CLI-based sessions. Plan Mode is a feature documented in GitHub's official changelog that allows developers to plan tasks before implementing them.

Changes:

• Introduces a new experimental configuration setting github.copilot.chat.cli.planMode.enabled (default: false) to enable Plan Mode for background agents
• Implements PlanAgentProvider, a new ChatCustomAgentProvider that registers a "Plan" custom agent by dynamically generating a .agent.md file in extension global storage
• Updates session handling to detect plan mode from request.modeInstructions2 and set the SDK session's currentMode property accordingly

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
src/platform/configuration/common/configurationService.ts	Adds new config key CLIPlanModeEnabled for the experimental plan mode setting
src/extension/chatSessions/vscode-node/copilotCLIPlanAgentProvider.ts	New file implementing the Plan agent provider with sentinel-based mode detection
src/extension/chatSessions/vscode-node/copilotCLIChatSessionsContribution.ts	Detects plan mode from request and passes it to session handler
src/extension/chatSessions/vscode-node/chatSessions.ts	Conditionally registers PlanAgentProvider when the feature flag is enabled
src/extension/agents/copilotcli/node/copilotcliSessionService.ts	Refactoring: simplifies LocalSessionManager initialization by removing try-catch and type alias
src/extension/agents/copilotcli/node/copilotcliSession.ts	Updates session to set currentMode to 'plan' or 'interactive' based on input
package.nls.json	Adds localization string for the plan mode setting
package.json	Adds the plan mode configuration setting with experimental tag

Comments suppressed due to low confidence (1)

src/extension/chatSessions/vscode-node/copilotCLIPlanAgentProvider.ts:14

• The prompt text contains "Github Copilot CLI" but should use "GitHub" (with capital H) for consistency with GitHub branding. This should be corrected to "GitHub Copilot CLI".

const planPrompt = `Plan model is configured and defined within Github Copilot CLI`;

[Copilot]

@DonJayamanne I've opened a new pull request, #3883, to work on those changes. Once the pull request is ready, I'll request review from you.