feat: Devil's Advocate (Challenger) agent charter template + auto-trigger

[tamirdresher]

Problem

Agents overstate findings and produce unchallenged analysis. In our squad, agents claimed 75-90% token savings when the real number was 20-55%. Without a challenger, wrong recommendations get implemented.

Proposal

Add an optional Challenger agent template to the framework:

• Charter template: .squad/templates/agents/challenger.md — role is to verify claims, run counter-hypotheses, check math, flag confidence levels (Verified/Unverified/Contradicted)
• Spawn integration: Coordinator can auto-spawn challenger before any architecture decision or when a claim exceeds a threshold (e.g., 'saves 75%')
• Iterative retrieval pattern: Max 3 investigation cycles, must cite evidence for every verdict
• Output format: Per-claim verdict table with evidence links

Prior Art

Field-tested as 'Q' agent across 200+ issues. Caught: inflated metrics, fabricated config references, wrong bottleneck assumptions. False positive rate ~15% (challenged things that were actually correct).

Deliverables

• Agent charter template
• Ceremony integration (auto-trigger before architecture decisions)
• Example spawn prompt in coordinator docs

[tamirdresher]

Revision after checking main: The Reviewer Rejection Protocol already exists in squad.agent.md — that covers post-work review. What's missing is a pre-decision challenger pattern: auto-spawning a fact-checker before architecture decisions or when agents make claims with specific metrics. The charter template + ceremony auto-trigger are the gap. Narrowing scope to those.

[bradygaster]

Resolved by #603 (merged). Challenger agent template + fact-checking skill are now available in templates/agents/challenger.md. Teams can adopt this pattern immediately.