add initial Go implementation of arcline-audit

Implements the full site health auditor with four check groups:

  - SSL/TLS (certificate validity, expiry, chain, TLS version, ciphers)

  - HTTP (redirect chain, security headers, response time)

  - DNS (A/AAAA, MX, SPF, DKIM, DMARC, DNSSEC)

  - Infrastructure (CDN detection, common port probes)

Includes CLI with --checks filter, --json and --out flags,

cross-compile Makefile, and GitLab CI pipeline.

Signed-off-by: Blake Ridgway <blake@blakeridgway.com>

INFRASTRUCTURE.md

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+# arcline-audit — Infrastructure & DevOps
+
+> **Full Site Health Auditor** — one-command SSL, HTTP headers, DNS,
+> redirects, and open ports check.
+
+---
+
+## Quick Reference
+
+| Attribute | Value |
+|-----------|-------|
+| **Type** | CLI tool |
+| **Tech** | Go (future) — reuses `arcline-check` internals |
+| **Deploy** | Static binary to `/usr/local/bin` |
+| **CI/CD** | ❌ Not yet created |
+| **State** | ⚪ Planned |
+
+---
+
+## Planned Checks
+
+| Check | Detail |
+|-------|--------|
+| **SSL/TLS** | Certificate validity, expiry, chain, TLS version, cipher warnings |
+| **HTTP** | Redirect chain, security headers (HSTS, CSP, X-Frame-Options), response time |
+| **DNS** | A/AAAA, MX, SPF, DKIM, DMARC, DNSSEC, rDNS |
+| **Infrastructure** | ASN/org lookup, CDN detection, common ports (80, 443, 22, 3306, 5432) |
+
+---
+
+## Deployment (Future)
+
+```bash
+# Install binary
+sudo cp arcline-audit-linux-amd64 /usr/local/bin/arcline-audit
+sudo chmod 0755 /usr/local/bin/arcline-audit
+
+# Run full audit
+arcline-audit example.com
+
+# Quick SSL check only
+arcline-audit --checks=ssl example.com
+
+# JSON output for programmatic use
+arcline-audit --json example.com > audit-report.json
+```
+
+---
+
+## Build (Template)
+
+```bash
+CGO_ENABLED=0 go build -trimpath -ldflags="-s -w" -o arcline-audit .
+```
+