Add configurable prompt file suggestions with when-clause support

[digitarald]

Summary

This PR introduces a new promptFilesRecommendations configuration setting that allows users to configure which prompt files should be suggested in the chat welcome view based on contextual when-clause expressions or simple boolean values.

cc @eli-w-king

Key Features

• Contextual suggestions: Configure prompt files to appear based on file type, language, or other VS Code context
• Flexible configuration: Support both boolean values (true/false) and when-clause expressions
• Smart caching: Async loading of prompt descriptions to avoid blocking UI rendering
• Unified experience: Suggestions now work in both experimental and regular welcome views

Configuration Example

{
  "promptFilesRecommendations": {
    "plan": true,                            // Always suggest
    "a11y-audit": "resourceExtname == .html", // Only for HTML files  
    "document": "resourceLangId == markdown", // Only for Markdown files
    "debug": false                           // Never suggest
  }
}

Implementation Details

Configuration Schema

• Added proper JSON schema with oneOf constraints supporting mixed boolean/string types
• Comprehensive validation and error handling for when-clause parsing
• Experimental tag with clear documentation and examples

UI Integration

• Enhanced ChatWidget with intelligent prompt suggestion logic
• Added description caching to avoid async calls during rendering
• Graceful fallback when descriptions fail to load

Welcome View Improvements

• Moved suggested prompts rendering outside experimental-only blocks
• Updated re-render logic to account for prompt suggestion changes
• Added helpful onboarding message when no agent-specific welcome exists

Testing

• ✅ Configuration parsing handles various input types correctly
• ✅ When-clause evaluation integrates properly with VS Code context system
• ✅ Error handling gracefully degrades on invalid configurations
• ✅ UI updates correctly when suggestions change

Related Issues

Enhances the reusable prompts feature by making suggestions context-aware and user-configurable.

Breaking Changes

None - this is an additive feature with sensible defaults.

[aeschli]

Can we use a different name than promptFilesSuggest. We use the term suggestions in the context of code completions.

Maybe 'recommendations'? Or 'favorites'?

• The setting allows to use context keys in user settings. That might be a first. Context keys are known to extension authors but typically not to regular users. We have a documentation if it, but it's in the extension section. There's no while editing in the settings editor and users might struggle to debug. Do we need the full power?

• Is there listening for prompt description changes or context changes?

[digitarald]

I added a link to the when clause in the settings. Non-extension devs users are also exposed to them for customizing keyboard shortcuts, which is a primary use case.

Given that this is meant to be a out of the box customization driven by power users for their other maintainers; it should be ok to not live-update descriptions, but biasing towards low performance impact and keep iterating on how this is being adopted.

[justschen]

just a thought, if we have way to categorize suggestions, these icons could maybe be different to reflect that, but not sure how possible that is.

[digitarald]

just a thought, if we have way to categorize suggestions, these icons could maybe be different to reflect that, but not sure how possible that is.

Havinc icons for prompts and modes would be neat, I agree. Locally I dropped in a play button and it makes it even more "clickable".