Guard against zero parsed steps in high-trust plan conversion

[williamthorsen]

Problem

The high-trust plan conversion in the orchestrator parses markdown ### Task N: sections (or reads a JSON companion) to produce orchestration-plan.json. If the resolved plan has zero steps — whether from an unexpected markdown structure or a malformed JSON companion — the empty steps: [] array is silently registered as a valid plan and forwarded to the coder.

Context

Found during review of #263 (plan provenance and trust-tiered orchestration). Classified as T1 (deferred TODO) since high-trust plans from known skills are very likely to have the expected structure, but the guard prevents silent failure in edge cases.

• File: packages/agents/content/skills/orchestrate/SKILL.md, "High-trust plan conversion" section
• The guard covers both resolution paths: JSON companion (step 1) and markdown parsing (step 2)

Solution

Add a zero-steps guard after plan content resolution in the "High-trust plan conversion" section. When steps is empty:

1. Emit a corrective phase_decision event for planning with run: true and reason explaining the fallback
2. Downgrade {planTrust} from "high" to "medium"
3. Abort the conversion — skip artifact writing and registration (steps 3-5)
4. Let Phase 2 (Planning) run with the medium-trust adoption-mode hint

This reuses existing medium-trust machinery rather than introducing new recovery logic.

Acceptance criteria

• Zero-steps guard is present after plan content resolution in SKILL.md
• Guard covers both JSON companion and markdown parsing paths
• Guard emits a corrective phase_decision event for planning
• Guard downgrades {planTrust} to "medium" and aborts artifact writing
• Existing high-trust conversion behavior is unchanged when steps > 0