Add explicit model routing for subagent dispatch

[WdBlink]

Summary

• add deterministic opc-harness model-route resolution before subagent dispatch
• preserve OPC's existing nodes, review counts, discussion rounds, and role topology
• default Claude Code routes to haiku for economy work and sonnet for standard work
• require explicit approval for inherit / opus premium routes
• allow repository and user config to replace tier values with any host-native model IDs
• wire the routing pre-flight into build, review, discussion, loop, and UX dispatch instructions

Motivation

Claude Code uses the parent model when a subagent model is omitted. In a multi-agent flow, that can silently fan a premium root model out across every implementation, test, observer, and review agent. The orchestration topology is valuable; the missing piece is an explicit, inspectable model-selection contract.

This change makes topology and model tier independent. A role or node resolves to economy, standard, premium, or none, then to a host-native model ID. Unknown routes fail closed instead of silently inheriting the root model. Claude Code's documented CLAUDE_CODE_SUBAGENT_MODEL override is honored and surfaced in the result.

Reference: https://code.claude.com/docs/en/sub-agents#choose-a-model

Public defaults and non-goals

• The built-in defaults target Claude Code's native haiku / sonnet aliases because OPC currently dispatches through Claude Code's Agent tool.
• Codex and other hosts can map the same tiers to their own model IDs through agentRouting.models; no flow definition needs to change.
• This does not add MiniMax, a local proxy, or any third-party-model assumption.
• This does not reduce OPC's evaluator count or bypass any quality gate.

Verification

• node --test bin/lib/model-routing.test.mjs — 15 passed
• related config and flow-core Node tests — 63 passed
• all 125 shell test suites under test/ passed (excluding the helper file)
• git diff --check
• node --check on the resolver and harness entrypoint
• npm pack --dry-run --json includes the resolver, tests, and routing protocol

Closes #32