feat(core): move timed-out shell commands to the background by default - #63699

Looks sound: the flag is gone and the plugin keeps a single code path. In every build, a foreground shell command still running after its timeout now moves to the background instead of being killed.

I checked this on the base by turning the flag off (OPENCODE_FLAGS=-shell_timeout_background, as in releases): the test "moves a foreground timeout to the background…" fails because the command is killed. With this PR the same test passes under the same setting. tool-shell.test.ts, job.test.ts and config normalization tests pass on Linux, and tsc is clean for core and schema.

Public API impact

• Removed the shell_timeout_background feature flag, so OPENCODE_FLAGS=±shell_timeout_background no longer has any effect.
• experimental.shell_timeout_background stays in the config schema as a deprecated, ignored key. Only its position moved in the OpenAPI document and the generated client types.
• Shell tool behaviour and description changed in release builds: timed-out foreground commands are backgrounded, and an explicit timeout on a background command is ignored.
• Existing configs keep loading. Users who relied on kill-at-timeout can no longer opt back into it.

One optional follow-up, outside the diff: the description of the timeout input (shell.ts:58) still says "Set to 0 to disable the timeout" and "Background commands have no timeout by default". That suggests an explicit timeout limits background commands, but it is now ignored for them. Rewording it would keep the model's instructions accurate.

Related: #61374

The new commits look good. The earlier optional point is addressed: the timeout input now describes a foreground wait that is ignored for background commands.

Dropping experimental.shell_timeout_background from the schema is safe for existing configs. A config that still sets it normalizes without diagnostics: the key is silently dropped and the rest of experimental is kept. At the new head, tool-shell.test.ts passes on Linux and tsc is clean for core and schema.

Public API impact

• Removed the shell_timeout_background feature flag.
• Removed the experimental.shell_timeout_background config key from the schema, OpenAPI and client types. Configs that set it still load.
• Changed the shell tool's timeout description and behaviour: it now only limits the foreground wait.

This is backwards compatible, but users can no longer opt back into kill-at-timeout.

This update looks good; I found nothing new to raise. After the rebase onto #63703, the change just removes the flag and applies the dev-build behaviour in every build. The earlier point about the timeout description is addressed.

I turned the flag off on the new base, as releases do: a foreground timeout still kills the command there, while this PR moves it to the background. tool-shell.test.ts and job.test.ts pass on Linux, and tsc is clean for core and schema.

Public API impact

• Removed the shell_timeout_background flag and the experimental.shell_timeout_background config key from the schema, OpenAPI and client types. Configs that set the key still load.
• In release builds, the shell tool's behaviour now matches dev builds:
  • Foreground commands move to the background when their timeout runs out.
  • Foreground timeout: 0 falls back to the 120 s default instead of waiting until the command finishes.
  • An explicit timeout on a background command still stops it.

This is backwards compatible, but users can no longer opt back into kill-at-timeout.