Parent: #1235
Blocked by: #1236

Summary

Implement LLM-based task decomposition — converts a user goal into a validated TaskGraph.

Branch: feat/m33/orchestration-planner

Deliverables

New files

• crates/zeph-core/src/orchestration/planner.rs — Planner trait + LlmPlanner impl

Details

• Prompt template with JSON schema and agent catalog injection
• String-based task_id in LLM output, parsed to internal TaskId(u32) indices
• JSON response parsing with retry-on-malformed (single retry)
• Validation of planner output via dag::validate
• max_tasks enforcement (reject if LLM exceeds limit)
• planner_model config: use configured model or fall back to agent's primary model
• planner_max_tokens config for response budget

Planner prompt design

Structured prompt includes:

1. Available agent catalog (name + description + tools)
2. JSON schema for output format
3. Instructions: prefer fewer/larger tasks, use string task_ids, declare dependencies

Tests (~15)

• Parse valid JSON with string task_ids into TaskGraph
• Reject JSON with cycles
• Reject JSON with invalid agent hints (warn but proceed with None)
• Reject JSON exceeding max_tasks
• Handle malformed JSON (non-JSON LLM response)
• Handle integer-based IDs (detect and reject with clear error)
• Empty goal handling
• Agent catalog formatting (correct prompt injection)
• Single-task goal produces single-node graph

Dependencies

Blocked by: Phase 1 (#1236)
Can run in parallel with: Phase 3 (#1238)