CR-129: Define external evaluator adapter contract

[coreytshaffer]

What changed

• Add external_evaluator_adapter_contract.v0 as a documentation-only boundary.
• Reserve a future closed-profile tc eval invoke-external shape without implementing a CLI or subprocess.
• Define mandatory evaluator identity and version, fixed bundle mapping, explicit cwd, result ownership, network posture, exit meanings, output encoding and caps, and process-tree timeout behavior.
• Require CR-128 bundle validation before any future process launch.
• Add drift tests proving no generic executable, free-form argv, CLI, or runtime adapter surface exists.
• Reconcile CR-128 status after PR CR-128: Validate evaluation handoff bundle integrity #101 merged and update the handoff docs, backlog, and changelog.

Why

The repository documents the external evaluator conceptually but does not yet own a versioned executable interface, bundle-aware argument contract, result destination, network posture, or portable termination behavior. Implementing a generic subprocess wrapper now would create an unsafe arbitrary-command surface. CR-129 records the prerequisites for a separately approved code-bearing slice.

Impact

This change adds no runtime behavior. TriageCore still does not launch evaluators, create or parse evaluator results, score cases, interpret exit statuses, write ledger evidence, or claim offline execution, approval, safety, or correctness.

Validation

• Focused contract tests: 10 passed.
• Full suite: 1002 passed, 3 skipped.
• git diff --check: clean.
• Runtime source contains no invoke-external command or external evaluator adapter module.