Issue 112: load workspace file directories lazily

[rawkintrevo]

Closes #112.

Summary:

• Lists workspace files one directory at a time using GCS delimiter-based listing and optional path query parameters.
• Loads file-tree children on folder expansion instead of relying on an eager recursive flat file list.
• Applies the same directory-scoped behavior to SSH-backed Dev machine file browsing.
• Updates docs and the workspace-files QA manifest for lazy folder expansion.

Tests:

• npm --prefix functions test
• npm run test:frontend -- workspaceFilesWorkflow
• npm --prefix session-runner run lint
• npm run docs:check
• npm run test:frontend
• npm --prefix session-runner test
• npm run build
• git diff --check

Browser QA:

• Case: e2e/qa/cases/workspace-files-editor.json
• Result: passed via Chrome DevTools against http://127.0.0.1:5173/app
• Evidence: artifacts/qa/workspace.files-editor/file-editor-lazy-download.png
• Report: artifacts/qa/workspace.files-editor/report.json
• Upload note: screenshot artifacts are local paths because this PR creation path does not upload binary screenshots.

Deployment:

• Functions changed, but production deploy was not run. The deploy command firebase deploy --only functions --project pi-agents-cloud was blocked by the approval reviewer because it mutates the shared Firebase/GCP project without explicit production-deploy approval.