Internal variable resolution service for skills

[pwang347]

Remove hardcoded references to /troubleshoot. Also, we should not use getCurrentCapturingToken as it can have timing issues. Fixes microsoft/vscode#302219

CC @aeschli

[Copilot]

Pull request overview

Introduces a centralized runtime variable-resolution service for skill templates (starting with {{CURRENT_SESSION_LOG}}) and wires it into skill file rendering/reading paths to avoid per-call-site placeholder handling.

Changes:

• Added ISkillVariableResolverService / SkillVariableResolverService to resolve well-known {{VARIABLE}} placeholders in skill content.
• Updated ReadFileTool and PromptFile to resolve skill variables generically for skill files (not just the troubleshoot skill).
• Updated prompt rendering DI to optionally provide IBuildPromptContext, and adjusted unit tests to cover broader skill variable replacement.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/platform/prompts/common/skillVariableResolverService.ts	New platform service that resolves skill template placeholders like {{CURRENT_SESSION_LOG}}.
src/extension/tools/node/readFileTool.tsx	Switches skill placeholder replacement logic to use the new resolver service for skill files.
src/extension/tools/node/test/readFile.spec.tsx	Updates tests to validate placeholder replacement for non-troubleshoot skills as well.
src/extension/prompts/node/panel/promptFile.tsx	Uses the resolver service when embedding skill files into prompts; derives session id from prompt context.
src/extension/prompts/node/base/promptRenderer.ts	Adds conditional DI registration of IBuildPromptContext based on root props.
src/extension/extension/vscode/services.ts	Registers the new ISkillVariableResolverService in the common service container.