Add optional Jev (TypeSafe System One) detector backend for complexity classification

[preved911]

Context

Raised in the chain-restructuring review (PR #17, "Think about jev usage for complicated commands detection"). Jev is a "System One" model: unstructured text in → typed structured decisions with calibrated probabilities out, ~70–500 ms, very cheap, schema outputs that cannot type-hallucinate.

Decision recorded in design.md: NOT in v1. The chain-restructuring change ships the deterministic AST-threshold detector only (per-line segment counts, nesting depth, inline-script statement counts). This issue tracks the follow-up.

Why a learned detector fits

The deterministic thresholds are brittle at the margins — "is this command too complex/unreadable to review?" is a fuzzy judgment. A structured-output classifier could:

• score commands where hand-written thresholds disagree with intuition (long pipelines, deeply nested but benign substitutions)
• drive smarter UX: reject only when complexity confidence is high; let borderline commands through to the normal human ask dialog

Proposed design

Optional, off-by-default detector backend in opencode-bash-guard.jsonc:

{
  "restructure": {
    "enabled": true,
    "detector": "jev"   // default "ast"
  }
}

• "ast" (default): current deterministic path — remains the source of truth and the fallback whenever the classifier errors or times out
• "jev": call the TypeSafe API with the command string, receive a structured decision ({ complex: boolean, confidence: number }-shaped schema), reject with the same actionable guidance when the decision is confidently "complex" and the chain resolves to ask

Constraints

• Privacy: commands leave the machine → strictly opt-in, documented in README
• Availability: on API failure/timeout → silent fallback to "ast" (never block, never throw)
• Determinism of the steering guarantee must not depend on the service
• Gated on Jev early-access availability and a stable API contract

Tasks

• Config: parse + validate detector field (unknown values → warn + "ast")
• Adapter: TypeSafe API client with timeout + typed response schema
• Fallback path: jev error/timeout → ast decision, no behavior surprise
• Tests with a mocked classifier (decide / pass / error / timeout)
• README: privacy note + config example

/cc PR #17 discussion.