fix: stream agent phase diagnostics live

[maxBRT]

Summary

• Streams Agent stdout while each Phase is still running, so curated tool and phase events reach the observer before process exit.
• Adds shared Agent subprocess supervision with timeout diagnostics, recent stderr capture, and Unix process-group termination for hung child processes.
• Rejects non-positive timeout values in .ship/config.yaml so Phase timeouts are not disabled by accident.

Fixes #76

Tests

• go test ./internal/agent ./internal/run -count=1
• go test ./...
• git diff --check

Notes

This keeps the terminal UI quiet during normal Phases. The useful live signal goes through the existing observer sink and durable Phase logs instead of printing raw vendor JSON.