Fix campaign metrics validation by documenting required fields in orchestrator instructions

[Copilot]

Campaign orchestrators were creating incomplete metrics snapshots, causing push_repo_memory.cjs validation failures: Metrics snapshot 'tasks_total' is required but was undefined.

Root Cause

The orchestrator instruction template only documented 2 of 4 required fields:

- Each snapshot MUST include `campaign_id` and `date` (UTC).

AI agents followed these incomplete instructions, writing metrics files like:

{
  "campaign_id": "docs-quality-maintenance-project73",
  "date": "2026-01-05"
}

The push_repo_memory.cjs validator expects all required fields per the CampaignMetricsSnapshot Go struct (which correctly lacks omitempty tags for required fields).

Changes

Updated orchestrator instructions (pkg/campaign/prompts/orchestrator_instructions.md):

• Document all 4 required fields explicitly
• Provide minimal valid snapshot example
• Clarify optional fields

- Each snapshot MUST include ALL required fields (even if zero):
  - `campaign_id` (string): The campaign identifier
  - `date` (string): UTC date in YYYY-MM-DD format
  - `tasks_total` (number): Total number of tasks (>= 0, even if 0)
  - `tasks_completed` (number): Completed task count (>= 0, even if 0)
- Example minimum valid snapshot:
  ```json
  {
    "campaign_id": "{{.CampaignID}}",
    "date": "2025-12-22",
    "tasks_total": 0,
    "tasks_completed": 0
  }

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