Issue 133: derive session target from workspace

[rawkintrevo]

Closes #133.

Summary

• Removed the session type chooser from the New session modal.
• Derive cloud runner vs SSH session payloads from the selected workspace source.
• Kept Cloud runner workspaces on the image selector and Dev machine workspaces on workspace-configured SSH sessions.
• Updated frontend smoke coverage, QA manifest expectations, and frontend architecture docs.

Tests

• npm run test:frontend
• npm run docs:check
• npm run build

Browser QA

• Case path: e2e/qa/cases/session-create-basic.json via scripts/open-session-modal.json modal path
• Result: passed
• Evidence paths:
  • artifacts/qa/session.create-basic/new-session-modal.png
  • artifacts/qa/session.create-basic/new-session-modal.snapshot.txt
  • artifacts/qa/session.create-basic/result.json

Screenshot upload limitation: screenshots were captured locally under artifacts/qa, but the available workflow here did not provide a GitHub screenshot upload mechanism.

Notes

• Browser QA observed the known local dev 404 for /__/firebase/init.json; QA login, Firebase token exchange, /api/me, /api/workspaces, and workspace data calls returned 200.

[github-actions]

Visit the preview URL for this PR (updated for commit 496e707):

https://pi-agents-cloud--pr134-133-remove-cloud-typ-aulhqhoe.web.app

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