feat(ai-powerups): manifest-based project context with read_project_file tool

[Pavel910]

Summary

• File manager: rename GetFileContents → GetFileContentsById, add new GetFileContentsByKey use case (S3 impl reads by key without KV metadata lookup), add ExtractFrontmatter handler that extracts description from .md/.mdx frontmatter on file upload
• AI context: replace inline file content in system message with a YAML-like manifest + read_project_file tool so the model fetches files on demand instead of receiving all content upfront
• Telemetry: generation results now include telemetry (filesRead, cacheHit, toolCallsMade, totalSteps) in the websocket response
• Refactor: extract ProjectFileCache and ProjectFileAssembler from AiPromptContextBuilder, reducing it from 4 deps to 2 and eliminating a 7-param buildContext method