feat: add URL title preview plugin

Event-driven plugin that auto-fetches page titles for URLs posted in
channel messages. HEAD-then-GET via SOCKS5 pool, og:title priority,
cooldown dedup, !-suppression, binary/host filtering. 52 tests.

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

docs/USAGE.md

@@ -50,6 +50,7 @@ channels = ["#test"]          # Channels to join on connect
 plugins_dir = "plugins"       # Plugin directory path
 rate_limit = 2.0              # Max messages per second (default: 2.0)
 rate_burst = 5                # Burst capacity (default: 5)
+paste_threshold = 4           # Max lines before overflow to FlaskPaste (default: 4)
 admins = []                   # Hostmask patterns (fnmatch), IRCOPs auto-detected
 timezone = "UTC"              # Timezone for calendar reminders (IANA tz name)
 
@@ -880,3 +881,46 @@ url = "https://paste.mymx.me"    # or set FLASKPASTE_URL env var
 
 Auth: place client cert/key at `secrets/flaskpaste/derp.crt` and `derp.key`
 for mTLS (bypasses PoW). Without them, PoW challenges are solved per request.
+
+### URL Title Preview (urltitle)
+
+Automatic URL title preview for channel messages. When a user posts a URL,
+the bot fetches the page title and description and displays a one-line
+preview. No commands -- event-driven only.
+
+```
+<alice> check out https://example.com/article
+<derp> ↳ Article Title -- Description of the article...
+```
+
+Behavior:
+
+- Automatically previews HTTP(S) URLs posted in channel messages
+- Skips private messages, bot's own messages, and command messages (`!prefix`)
+- URLs prefixed with `!` are suppressed: `!https://example.com` produces no preview
+- HEAD-then-GET fetch strategy (checks Content-Type before downloading body)
+- Skips non-HTML content types (images, PDFs, JSON, etc.)
+- Skips binary file extensions (`.png`, `.jpg`, `.pdf`, `.zip`, etc.)
+- Skips FlaskPaste URLs and configured ignore hosts
+- Dedup: same URL only previewed once per cooldown window (5 min default)
+- Max 3 URLs previewed per message (configurable)
+- Title from `og:title` takes priority over `<title>` tag
+- Description from `og:description` takes priority over `<meta name="description">`
+- Title truncated at 200 chars, description at 150 chars
+
+Output format:
+
+```
+↳ Page Title -- Description truncated to 150 chars...
+↳ Page Title
+```
+
+Configuration (optional):
+
+```toml
+[urltitle]
+cooldown = 300        # seconds before same URL previewed again
+timeout = 10          # HTTP fetch timeout
+max_urls = 3          # max URLs to preview per message
+ignore_hosts = []     # additional hostnames to skip
+```