Enhance existing workflows, add 3 new workflows, CI, prompts, and scripts

[23seriy]

What

Extends the repo from 14 to 17 workflows, improves 4 existing workflows based on real-world usage, adds 2 prompts, 2 scripts, GitHub Actions CI, and a CHANGELOG.

New workflows

Workflow	Domain	Description
/helm-chart-review	kubernetes	Review Helm charts for security, reliability, and best practices: resource specs, probes, security context, PDBs, anti-affinity, RBAC
/secrets-leak-scan	security	Scan git repo history for leaked secrets using gitleaks, trufflehog, or regex fallback
/incident-triage	observability	Guided first 15 minutes of a production incident: timeline, blast radius, evidence gathering, mitigation suggestions

Improved workflows

Workflow	Improvement
/aws-account-audit	Added FAST=yes input to skip slow per-policy IAM loops on large enterprise accounts (>1000 roles). Restructured Step 3 with bulk checks always + deep scan opt-in
/aws-cost-quickscan	Added DEEP=yes input and new Step 9 for per-instance EC2/RDS CPU utilization analysis via CloudWatch
/terraform-plan-review	Added Step 0 with plan generation commands (text, JSON, binary, Terragrunt) for users who don't have a plan yet
/k8s-debug	Enhanced log analysis (init containers, structured error extraction, severity classification, "noisiest pods" scan), added restart timeline analysis, HPA/autoscaling health check, expanded triage cheat-sheet

New prompts

• pr-description.md — generate PR descriptions from diffs
• explain-like-a-senior.md — explain infrastructure code to junior engineers

New scripts

• aws-whoami.sh — quick AWS identity + account context check
• stale-branches.sh — list git branches older than N days

Repo polish

• GitHub Actions CI: markdown lint, link check, frontmatter validation, README link verification, shellcheck
• CHANGELOG.md with full project history

Testing

• /aws-account-audit FAST mode logic verified against real run data from account 302584366770 (4,572 roles, 2,261 policies)
• /aws-cost-quickscan DEEP mode commands tested for syntax correctness
• /k8s-debug improvements based on real cluster debugging sessions (RKE2 clusters)
• All workflows lint-clean with valid frontmatter

Risk

• Risk level: Low — all changes are additive (new files + non-breaking improvements to existing workflows)
• Rollback: Revert the merge commit
• No breaking changes to existing workflow interfaces