feat: add env-example-generator skill and secure-config-advisor agent

[ArthurAdrianoMM]

What

Adds the env-example-generator skill and the secure-config-advisor agent.

• Skill: scans the codebase (app code, Dockerfiles, compose, CI, K8s manifests) for every environment variable actually read, then generates a documented .env.example with grouped, commented placeholders - and reports drift between code and the example file. Never copies real secret values.
• Agent: a configuration security persona covering secret hygiene, secret manager adoption, fail-fast config validation, and 12-factor config separation.

Why

Out-of-date .env.example files are a universal onboarding pain, and hand-maintained ones drift immediately. Existing security skills detect leaked secrets; this pair complements them on the prevention/documentation side.

Checklist

• npm run skill:validate passes for the new skill (note: an unrelated pre-existing skill aws-cloudwatch-investigation currently fails validation on staged)
• npm run build executed to update the README
• No duplicate of existing skills/agents