LLC adapters: no read-only sign-in probe before dispatch — signed-out agents burn scheduler slots #13098

Problem

Nothing checks whether an agent's CLI is signed in before the scheduler
dispatches a run to it. A signed-out agent is dispatched anyway, spawns, fails,
and is diagnosed only after the fact by keyword-scanning its output tail. The
next heartbeat re-dispatches it and the cycle repeats.

The result is a silent, self-repeating burn of scheduler slots and agent budget,
surfaced to the operator only as generic run failures with no actionable cause.

Evidence

The only pre-dispatch auth check in the entire adapter layer:

• autobot-backend/llc/adapters/copilot_subscription_adapter.py:91-107 — and it
  works by spawning gh auth status and awaiting it, i.e. a process spawn on
  every single invoke.

No check at all in:

• llc/adapters/claude_code_adapter.py
• llc/adapters/claude_code_subscription_adapter.py
• llc/adapters/codex_subscription_adapter.py
• llc/adapters/copilot_local_adapter.py

Nothing gates dispatch in the scheduler:

$ grep -rn --include=*.py "should_skip\|can_dispatch\|precheck\|_eligible" autobot-backend/llc/scheduler/
(no output)

Failure is instead classified post-hoc by is_rate_limit_output()
(llc/adapters/subprocess_support.py:56-70), a keyword match over error text.

Proposed approach

Add a declarative, read-only, zero-spawn credential probe per adapter —
the counterpart to the existing resolve_cli_binary() 3-tier search
(llc/adapters/subprocess_base.py:66-105), which answers "is it installed".
This answers "is it usable".

Probe kinds, as data rather than per-adapter code:

Wire it into the heartbeat scheduler as a pre-dispatch gate.

Hard constraint — fail OPEN, not closed

A failed or inconclusive probe must warn and dispatch anyway. It must never
block a run.

These probes read third-party CLIs' private on-disk credential formats, which
change without notice. A probe that wrongly reports "signed out" and blocks
dispatch is strictly worse than having no probe at all, because the scheduler is
unattended — there is no human present to notice and override.

Acceptance criteria

• Declarative probe definition per adapter (data, not bespoke code per agent)
• Probe performs no process spawn and never reads secret material
• Scheduler logs an actionable reason (e.g. "agent CLI not signed in — run
  <login command>") and records it on the work item
• Probe failure never blocks dispatch (explicit test)
• copilot_subscription_adapter's spawn-based check is replaced by the
  read-only probe
• Tests cover: signed-in, signed-out, and probe-error (inconclusive) paths

Notes

Found during a capability audit of the LLC adapter layer.