Issue 115: sync uploaded files to active runners

[rawkintrevo]

Summary

• Add an authenticated POST /api/workspaces/{workspaceId}/sync-files route that asks running cloud sessions to sync workspace files from Cloud Storage into their live /workspace directory.
• Add a protected runner POST /workspace/sync-down endpoint used by Functions after file-browser uploads and editor saves.
• Trigger sync after both normal API uploads/saves and large direct Firebase Storage uploads.
• Update developer wiki notes for the backend and runner sync behavior.

Closes #115

Tests

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

Deploy / Runtime Notes

• Deployed Cloud Functions with firebase deploy --only functions --project pi-agents-cloud; api updated to revision api-00122-nub.
• This changes session-runner/server.js; affected runner images need rebuilt revisions, and existing Cloud Run sessions need restart/recreation before they expose /workspace/sync-down.

QA

• Browser QA not run by agent per user request: user will QA upload -> terminal visibility.
• No QA screenshots produced by agent.

[github-actions]

Visit the preview URL for this PR (updated for commit 79b74e1):

https://pi-agents-cloud--pr128-115-uploaded-files-t-1c1crdfd.web.app

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