Resolve selected capability roots without starting executors

[jif-oai]

Why

A thread can select skill roots that live in an executor environment. skills/list needs a passive snapshot of the roots that are usable now: it must not start an executor, wait for recovery, or reconnect a failed environment.

The initial implementation checked the immutable first startup result. After a successful connection later entered recovery or failed, that result still looked successful. A read-only catalog request could then wait for recovery or trigger a new connection while reading the filesystem.

What

• inspect readiness from the current exec-server connection state
• return roots only while their environment can serve a request immediately
• omit environments that have not started, are connecting, or are recovering
• return warnings for missing environments and terminal connection failures
• add a fail-fast filesystem view that never starts, waits for, or reconnects an environment
• expose the passive selected-root snapshot through CodexThread

Behavior

• Local and currently connected environments are ready.
• Starting and recovering environments are omitted without a warning so callers can retry later.
• Missing and terminally failed environments are omitted with a warning.
• A disconnect between readiness inspection and filesystem access fails promptly instead of crossing into the normal recovery path.
• Normal model-turn and execution paths keep their existing reconnect behavior.

Design

The recovery policy is private to the exec-server client. Callers choose the explicit fail-fast filesystem method; the existing client and filesystem APIs remain reconnecting. This keeps the passive contract at the transport boundary instead of plumbing timeout or retry flags through the skills stack.

Coverage

• a lazy stdio environment stays unstarted during passive inspection
• missing and terminally failed environments surface warnings
• a real websocket disconnect proves current readiness drops, a previously acquired fail-fast filesystem handle returns promptly, and readiness returns after recovery

Scope

This PR only provides passive readiness and fail-fast filesystem primitives. It does not add app-server API fields or notifications.

Stack

• Expose thread-selected skills from skills/list #31582 uses these primitives for experimental thread-scoped skills/list.
• Notify clients when thread-selected skills change #30228 adds targeted invalidation notifications.

[chatgpt-codex-connector]

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