feat: add wave 4 plugins (opslog, note, subdomain, headers)

Opslog: timestamped operational log per channel with add, list,
search, and delete. SQLite-backed, admin-only clear.

Note: persistent per-channel key-value store with set, get, del,
list, clear. SQLite-backed, admin-only clear.

Subdomain: enumeration via crt.sh CT log query with optional DNS
brute force using a built-in 80-word prefix wordlist. Resolves
discovered subdomains concurrently.

Headers: HTTP header fingerprinting against 50+ signature patterns.
Detects servers, frameworks, CDNs, and security headers (HSTS, CSP,
XFO, etc).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/USAGE.md

@@ -90,6 +90,10 @@ level = "info"                # Logging level: debug, info, warning, error
 | `!tor <ip\|update>` | Check IP against Tor exit nodes |
 | `!iprep <ip\|update>` | Check IP against Firehol/ET blocklists |
 | `!cve <id\|search>` | CVE lookup from local NVD mirror |
+| `!opslog <add\|list\|search\|del\|clear>` | Timestamped operational log |
+| `!note <set\|get\|del\|list\|clear>` | Per-channel key-value notes |
+| `!subdomain <domain> [brute]` | Subdomain enumeration (crt.sh + DNS) |
+| `!headers <url>` | HTTP header fingerprinting |
 
 ### Command Shorthand