feat(turn): report the managed executor and fail closed when it cannot launch

[huangruiteng]

Summary

A planned managed Turn now reports which executor it would run on and fails
closed when that executor provably cannot launch here. Stacked on the
credential-resolved default Turn host change.

Changes

• managed_executor_binding projects the executor, its kind (managed,
  individual, generic), the credential env var name (never its value), a
  launchability verdict, and a typed reason when unavailable.
• loopx turn plan and loopx turn run-once attach managed_executor to their
  payload.
• The Turn executor stops before any effect when the planned executor is
  unlaunchable: status unavailable, typed reason, no journal write, no quota
  spend.
• turn plan default execution mode follows the resolved host, so a managed
  default plans isolated-headless instead of an execution mode the outer
  controller rejects.

Validation

• tests/test_turn_managed_executor_binding.py, executor fail-closed and
  plan-preview tests, tests/test_turn_default_host_binding.py CLI default-mode
  test; 93 focused tests then 290 across the turn driver/executor suites, green.
• examples/loopx-turn-managed-executor-binding-smoke.py proves the four-step
  CLI behavior, including an import-blocked dsh runtime.
• Live CLI readback (no provider calls): no credential -> codex-cli /
  individual; credential -> dsh / managed / available: true; blocked
  runtime -> dsh / available: false / dsh_runtime_unavailable.

Boundaries

• No credential values are read or projected; only env var names appear.
• No provider calls are made by tests or the smoke.

Review notes

• Interface budget: loopx turn plan gains the semantic field
  managed_executor, attributed as managed_executor_binding_v0. The
  agent-facing CLI base/head differential grants one bounded, one-time growth
  allowance (512 chars / 512 bytes / 12 lines / 448 compact chars) bound to the
  declared none-to-v0 binding transition on loopx_turn_plan,
  loopx_turn_plan_transaction_detail and loopx_turn_run_once_preview only.
  Quota, status and every other agent-facing surface keep their ordinary budget,
  and a v0-to-v0 change receives no allowance.
• Module budget: the fail-closed decision and the payload projection moved to
  host_binding.py, the binding owner, so the already-large
  turn_driver/executor.py does not grow (1507 -> 1496 lines; limit 1500).
• Validation: tests/canary/test_maintainability_ratchet.py 8 passed;
  tests/control_plane/test_cli_output_differential.py 59 passed;
  examples/control_plane/cli-output-base-head-differential-smoke.py ok
  (base=102 candidate=102 review_required=0); 81 focused Turn tests passed.

[huangruiteng]

Superseded by #4443 (feat(turn): select the managed Turn host explicitly).

The value here is kept: managed_executor_binding, the managed_executor readback block on plan and run-once, the typed unavailable_reason values, and the fail-closed start (status unavailable, no host invocation, no journal write, no quota spend). The readback now reports the executor the operator selected rather than the one a credential implied, and operator_credential_bound is stated as the narrower claim it is.

#4443 is main-based and carries the CLI output allowance this PR also needed, so the two no longer have to be reviewed as two layers. Closing to compress the delivery chain.