refactor(tui): remove client-side derived state, use daemon-computed fields

[proboscis]

Summary

• Add alive, alive_known, is_active, and pr_status fields to Run proto message
• Daemon now computes these values server-side instead of TUI deriving them locally
• Remove derive_pr_status() function and is_active() method from Python TUI
• Simplify elapsed_time() to use daemon-provided elapsed_display

Changes

Proto Schema (api/orch.proto)

• Added 4 new fields to Run message: alive, alive_known, is_active, pr_status

Go Daemon (internal/daemon/proto_handler.go)

• Added protoIsActiveStatus() to compute active state from status
• Added computePRStatus() to derive PR status from run state
• Added computeAliveStatus() to check agent liveness via multiplexer
• Updated enrichment functions to populate new fields

Python TUI

• models.py: Added fields, removed is_active() method, simplified elapsed_time()
• proto_client.hy: Map new proto fields to model
• widgets.py: Removed derive_pr_status(), use run.pr_status directly
• tests/test_widgets.py: Updated tests for field-based approach

Testing

• Go tests pass (2 pre-existing integration test failures unrelated to changes)
• Python TUI tests pass (83 passed, 9 failed due to pre-existing hy module issue)

Closes orch-392