Delete the legacy harness execution path once jobs run as T3 threads #110

Outcome

Model Router runs every job through T3 threads only. The legacy execution path, in which the runner launches and resumes harnesses itself, is deleted once the T3 path (#106) is proven by one real job.

Why

The user confirmed on 2026-09-24 that depending on T3 is fine (they are the only user), and the Mission now says planners are hosted in T3 and jobs run as T3 threads. Keeping both paths doubles maintenance, and several of today's failures live only in the legacy path: the concurrent OpenCode server port race (#104), the four harness-specific planner callbacks (#94, #96), and resuming a live planner session in a second process.

Delete (after #106 is live)

• Harness launch and resume code in runner/harnesses.py and most of runner/adapters.py: Codex CLI and app-server launch, owned opencode serve, Grok Build CLI, Claude resume.
• The four planner callback kinds and their busy-detection code.
• The owned-OpenCode kit config directories, where T3 provider-instance settings replace them (verify first).
• Close feat: native Codex dispatcher transport with attachable loopback endpoint (#86) #102 (native Codex dispatcher transport) and Concurrent OpenCode worker starts: control times out while the session keeps working #104 as superseded.

Keep

policy.py routing, capacity probes and ledger, the controller and job ledger, dispatcher envelope handling, stall and capacity classification (fed by T3 activities), and Observer capture.

Acceptance criteria

• Every stage (dispatch, worker, correction, recovery, planner question, terminal report) runs through T3 threads; submit requires a T3 planner thread.
• python3 scripts/test.py passes with the deleted code and its tests removed, and no unique regression proof is lost for behavior that remains.
• One real job through the installed release still reaches an open PR.

Blockers

#106 merged and proven live with one real job.

Objective contribution

Simplifies the milestone's delivery path to one executor; removes the failure sources seen on 2026-09-25.