fix: honor agent model routing

[mathewtbenjamin]

Summary

Sub-agents declaring a model family in frontmatter (haiku, sonnet, opus) previously fell through to the parent model — the declaration was silently ignored. This PR makes agent-runner resolve the declared family against the authenticated Pi model registry:

• Resolution prefers the parent's provider when it offers that family, then direct Anthropic, then Anthropic Vertex, choosing the newest matching model by version.
• Unresolvable declarations fall back to the parent model (previous behavior, now explicit).
• prompt.md transcripts record both the requested and resolved model for auditability.

Also included (surfaced while testing the routing paths):

• Provider policy refusals explicitly flagging cybersecurity content are treated as non-retryable — retrying an unchanged phase prompt can't succeed and wastes requests. The refusal is still recorded in audit-state.json so the operator can resume with another model.
• Replaced a Model<any> + biome-ignore with Model<Api>.

Tests

• test/agent-runner.test.ts: family resolution (provider preference, version ordering, fallback).
• test/retry.test.ts: non-retryable refusal classification.

Docs updated in HACKING.md.