Reuse Session executors through a shared Harness contract

[SaladDay]

A Session now retains its native executor between Turns. Core schedules durable work through one Runtime lifecycle; Codex, Claude and MiniMax implement the same Executor/Turn contract. Healthy follow-up Turns preserve their native process and exact history.

• Separate preparation, admission, cancellation, native settlement, resource cleanup and idle expiry. Retain cleanup ownership on uncertainty and never replay uncertain input. Temporary preparation capacity leaves durable work queued for the existing scheduler.
• Use the common lifecycle for connected managed and self-hosted environments. Closing an executor does not reclaim its environment or delete its workspace.
• Confirm native work has stopped before reporting cancellation. Claude distinguishes unanswered function calls from submitted results awaiting receipts. Codex terminates its thread-owned background terminals through confirmed native operations.
• Notify the existing scheduler after input commits and record readiness, start-control and first-text timings separately. Replace the private wire/bridge contracts without compatibility paths.
• Link the Harness onboarding guide from README, covering interfaces, registration, optional capabilities, qualification and a runnable example. Consolidating these declarations into a single onboarding entry file remains a separate task.

Validation:

• Full make check, with the unchanged Web gate executed on mx and other gates on zju. Includes the dedicated PostgreSQL suite, generated-contract checks, 55 browser tests, Claude bridge tests and Rust checks.
• Full adapter/Runtime tests and focused race regressions. Final independent blind review of 2064a177..fecf58da found no substantiated in-scope issue.
• Real pinned official SDK plus raw HTTP against all three native engines: standalone baseline/candidate runs and managed Docker/self-hosted Docker placement runs. Covers normal reuse, restart/history, cancellation, continuation, Files/Artifacts and isolation. Final Codex runs verify the cancelled foreground command stops producing side effects in both placements; final Claude functions run verifies cancellation while awaiting an unanswered function.

Native versions: Codex 0.153.4, Claude SDK 0.3.269 / native 2.1.269, MiniMax 0.4.12. Versioned evidence is retained under /home/fanjingluo/.oac/acceptance/session-executor-reuse-20260928 on zju, including failed runs and final cleanup records. The earlier MiniMax managed sample contained a native duplicate tool call; the one permitted fresh-Session strict retry passed.

Timing samples prove retained ownership and avoided native preparation, not a universal response-time improvement. Queue and environment-ready durations cannot be separately derived from the standalone samples; this limit is preserved in the evidence.