feat(headless): bind a persistent PTY session and stdin writes to the benchmark tool surface #2242

Problem

The headless benchmark surface exposes only a blocking, one-shot Bash (buildForegroundBashTool): the call returns when the command exits or the 120s timeout kills it. There is no session handle, no stdin, no mid-flight output. Any task needing an interactive process — a REPL, a debugger, an installer prompt, a booting VM — is unreachable, and any long command is all-or-nothing with no way to observe or intervene.

Codex's exec_command returns after a 10s yield with a session_id while the process keeps running, and write_stdin writes to or polls that session. Across the same 89-task Terminal-Bench 2.1 benchmark Codex issued 381 write_stdin calls; on qemu-alpine-ssh it was 22 exec_command paired 1:1 with 22 write_stdin.

Desired outcome

The headless surface can start a process, get a handle back before it finishes, read incremental output, and write to its stdin — so interactive tasks are representable and long commands are observable.

Alternatives or workarounds

packages/runtime/src/shell-tools.ts already has buildManagedBashTool with background and PTY support; the headless path simply binds the foreground variant instead (packages/headless/src/tools.ts:162). The gap is wiring, not a missing capability. The workaround the model currently discovers on its own is nohup … &, which leaves orphan processes alive into the grading phase.

Evidence