Fix Codex-host autoplan outside voice routing

[Spenquatch]

Summary

• make /autoplan dual-voice instructions host-neutral around subagent vs outside voice
• add a host-aware autoplan resolver that keeps Codex as the outside voice on Claude/non-Codex hosts and flips Codex hosts to claude -p with read-only file tools
• add regression coverage for generated Codex and Claude autoplan outputs

Validation

• bun run scripts/gen-skill-docs.ts --host codex --dry-run
• bun run scripts/gen-skill-docs.ts --dry-run
• bun test test/gen-skill-docs.test.ts

Notes

• Codex-hosted autoplan no longer emits Claude-first subagent wording or nested Codex outside-voice blocks.
• Claude-hosted autoplan keeps Codex as the outside voice, but the generated copy now uses neutral subagent/outside voice terminology in the dual-voice flow.

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