chore: campaign discovery via label-based approach

[Copilot]

Campaign discovery relied on ephemeral cache memory files, causing reliability issues across distributed executions. This replaces it with persistent GitHub labels.

Label System

Two-tier labeling for campaign items (issues, PRs, discussions):

• agentic-campaign - Generic marker for all campaign content. Prevents non-campaign workflows from processing these items.
• z_campaign_<id> - Campaign-specific identifier (e.g., z_campaign_security-q1-2025). Enables precise per-campaign queries.

Discovery Priority

// 1. Primary: Campaign-specific label
const campaignLabel = `z_campaign_${campaignId}`;
const result = await searchByLabel(campaignLabel, repos, orgs);

// 2. Secondary: Generic campaign label  
if (result.items.length === 0) {
  await searchByLabel("agentic-campaign", repos, orgs);
}

// 3. Fallback: Cache memory (deprecated, logs warning)
// 4. Fallback: Tracker-ID search

Changes

Infrastructure (Go)

• Added label constants: AgenticCampaignLabel, CampaignLabelPrefix
• Added FormatCampaignLabel() helper with sanitization (spaces → hyphens, lowercase)

Discovery (JavaScript)

• Refactored campaign_discovery.cjs to prioritize label searches
• Deprecated discoverFromMemory() with warnings

Templates

• Epic issues now automatically receive both labels
• Added "Campaign Label Requirements" documentation
• Documented skip-if-match pattern for non-campaign workflows

Workflow Isolation

Non-campaign workflows filter out campaign items:

on:
  issues:
    types: [opened, labeled]
    skip-if-match:
      query: "label:agentic-campaign"
      max: 0

Backward Compatibility

Cache memory and tracker-ID discovery remain as fallbacks. No breaking changes.

Original prompt

Don't rely on cache memory for campaign discovery but use labels. Each campaign issue (epic or worker) should get a label "agentic-campaign

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