fix: prioritize PR status over waiting input

[cneuralnetwork]

Summary

Prioritize PR-derived session statuses over a sticky waiting_input activity state so workers that have opened PRs show the actionable PR pipeline state instead of remaining pinned to needs_input.

Root Cause

deriveStatus returned needs_input before evaluating PR facts. If a worker's last persisted activity signal was waiting_input, newer PR facts such as an open PR or failing CI could not affect the display status.

Changes

• Evaluate open and merged PR facts before falling back to waiting_input for live sessions.
• Update status derivation tests for waiting_input combined with open, failing, and absent PR facts.
• Refresh the architecture status precedence diagram to match the read model.

Testing

• git diff --check
• GOTOOLCHAIN=auto GOSUMDB=sum.golang.org go test ./internal/service/session
• Attempted GOTOOLCHAIN=auto GOSUMDB=sum.golang.org go test ./...; it hit unrelated local terminal/runtime failures in backend/internal/adapters/runtime/tmux (no server running on /tmp/tmux-1000/default) and backend/internal/terminal (fish-shell parsing during an attachment integration test). The changed session package passed.

Fixes #2174