chore(EC-1851): trim AGENTS.md and add design docs per agentic SDLC best practices

[joejstuart]

Summary

• Reduces AGENTS.md from 243 to 75 lines following wg-agentic-sdlc repo scaffolding best practices
• Removes content agents discover on their own: cmd/ listing, internal/ package descriptions, rule filtering details that duplicated .cursor/rules/*.mdc
• Keeps only what prevents mistakes: test tags with timeouts, acceptance test flags, CGO/DNS issue, /etc/hosts, macOS Podman setup, debug mode, troubleshooting
• Adds internal/evaluator/DESIGN.md — why two PolicyResolvers, two-pass filtering design, scoring precedence, how to add filters
• Adds internal/validate/vsa/DESIGN.md — VSA purpose, storage backend rationale, DSSE signing decision, expiration model
• Design docs placed alongside code per Tier 2 best practices (not in .cursor/rules/)

Jira: https://redhat.atlassian.net/browse/EC-1851

Test plan

• Verify make ci still passes (no code changes)
• Confirm AGENTS.md is under 150-line target
• Validate DESIGN.md content against current source (no stale references)

🤖 Generated with Claude Code

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Enterprise

Run ID: b101e215-cba4-45ad-b908-128ae6ff19a4

📥 Commits

Reviewing files that changed from the base of the PR and between 8aec709 and d374871.

📒 Files selected for processing (3)

• AGENTS.md
• internal/evaluator/DESIGN.md
• internal/validate/vsa/DESIGN.md

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Flag	Coverage Δ
acceptance	55.60% <ø> (+<0.01%)	⬆️
generative	17.82% <ø> (ø)
integration	26.56% <ø> (ø)
unit	69.04% <ø> (ø)

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