feat: select subagent models per task

[davidrobertson]

Summary

• Replace fixed model and reasoning pins with task-specific selection from live Codex capabilities.
• Choose each subagent configuration from task difficulty, ambiguity, risk, and verifiability.
• Preserve explicit user selections and report degraded routing instead of silently downgrading.
• Reject ultra reasoning, including profile and parent-inheritance fallbacks; refuse the spawn when no compliant route exists.

This supersedes #9. Routine reviews are no longer pinned to Terra high.

Validation

• npm test: 48/48 tests passed in the source checkout.
• npm test: 48/48 tests passed in the installed plugin copy.
• git diff --check main...HEAD passed.
• Installed policy files match commit 3df09a0 byte-for-byte.
• Independent composite review passed.