Allow Mermaid diagrams in Plan mode

[OrenMe]

This PR enables Mermaid diagrams in Plan mode behind an explicit configuration gate and updates the Plan prompt so Mermaid diagrams are rendered consistently when they materially help a plan.

Key changes:

• Add a Plan-specific Mermaid setting and expose it in the settings UI
• Gate the Mermaid render tool and Plan prompt instructions behind that setting
• Restore the Plan style-guide wrapper and keep Mermaid instructions concise and explicit
• Add regression coverage for the Plan agent generated content
• Update the switch-to-Plan path so it reflects the same Plan-mode Mermaid guidance

Refs microsoft/vscode#307804.

[Copilot]

Pull request overview

Enables the Plan agent to include Mermaid diagrams (and use the Mermaid render tool) when explicitly enabled via configuration, keeping Plan-mode output rules consistent across agent generation and the switch-to-Plan tool.

Changes:

• Add chat.planAgent.mermaid.enabled / github.copilot.chat.planAgent.mermaid.enabled configuration and localized description.
• Gate Plan agent body instructions + tool list to include Mermaid guidance and the Mermaid render tool only when enabled.
• Add/extend PlanAgentProvider regression tests around Mermaid gating and style-guide rules.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
src/platform/configuration/common/configurationService.ts	Adds a Plan-specific Mermaid enablement config key.
src/extension/tools/vscode-node/switchAgentTool.ts	Switch-to-Plan message now reflects Mermaid guidance based on the setting.
src/extension/agents/vscode-node/planAgentProvider.ts	Conditionally injects Mermaid rules + tool, and updates Plan style guide wrapper/rules.
src/extension/agents/vscode-node/test/planAgentProvider.spec.ts	Adds regression coverage for Mermaid gating + Plan output rules.
package.nls.json	Adds localized setting description.
package.json	Exposes the new setting and also changes build scripts to use tsx.