docs: document username plugin

Add !username section to USAGE.md with examples. Add OSINT quick
reference entries to CHEATSHEET.md. Mark username plugin done in
ROADMAP.md and TASKS.md.

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

docs/USAGE.md

@@ -112,6 +112,9 @@ format = "text"               # Log format: "text" (default) or "json"
 | `!payload <type> [variant]` | Web vuln payload templates |
 | `!dork <category\|list> [target]` | Google dork query builder |
 | `!wayback <url> [YYYYMMDD]` | Wayback Machine snapshot lookup |
+| `!username <user>` | Check username across ~25 services |
+| `!username <user> <service>` | Check single service |
+| `!username list` | Show available services by category |
 
 ### Command Shorthand
 
@@ -444,3 +447,39 @@ Check the Wayback Machine for archived snapshots of a URL.
 
 Auto-prepends `https://` if no scheme is provided. Uses the Wayback Machine
 availability API.
+
+### `!username` -- Username Enumeration
+
+Check username availability across ~25 services using HTTP probes and
+public JSON APIs.  Supports GitHub, GitLab, Reddit, Docker Hub, Keybase,
+Dev.to, Twitch, Steam, and more.
+
+```
+!username list                     List services by category
+!username john                     Full scan (~25 services)
+!username john github              Check single service
+```
+
+Output format:
+
+```
+Full scan:
+  Checking "john" across 25 services...
+  john -- 8 found, 14 not found, 3 errors
+  Found: GitHub, GitLab, Reddit, Twitch, Steam, PyPI, Docker Hub, Medium
+
+Single service:
+  GitHub: john -> found | https://github.com/john
+  GitHub: john -> not found
+
+List:
+  Dev: GitHub, GitLab, Codeberg, ...
+  Social: Reddit, Twitter/X, ...
+  Media: Twitch, Spotify, ...
+  Other: Steam, Pastebin, ...
+```
+
+- Username must match `[a-zA-Z0-9._-]{1,39}`
+- Full scan sends acknowledgment before probing
+- 8 parallel workers, 20s overall timeout
+- Three check methods: HTTP status, JSON API, body search