feat: Discord bot — per-session rules, user identity, and DISCORD_CLAUDE.md

- Add DISCORD_CLAUDE.md as the Discord bot's dedicated system prompt,
  replacing the main CLAUDE.md for bot sessions. Covers: no-interactive
  rules, Discord user authorization, vault/remediation guidance, /save
  after every task, and formatting rules for Discord.

- config.py: add discord_system_prompt field (default: projects/discord-bot/
  DISCORD_CLAUDE.md, overridable via env var).

- client.py: _load_system_prompt() now loads discord_system_prompt path
  with fallback to CLAUDE.md if file is missing.

- message_handler.py: inject [DISCORD_CONTEXT] header into every agent
  message containing Discord username, display name, user ID, channel,
  and guild so the agent always knows who is asking.

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

projects/discord-bot/bot/handlers/message_handler.py

@@ -29,21 +29,37 @@ class MessageHandler:
         if message.author == self.bot.user:
             return
 
-        content = message.content
+        # Strip the bot mention to get the raw user text.
+        user_text = message.content
         for mention in message.mentions:
             if mention == self.bot.user:
-                content = content.replace(f"<@{mention.id}>", "").replace(
+                user_text = user_text.replace(f"<@{mention.id}>", "").replace(
                     f"<@!{mention.id}>", ""
                 ).strip()
 
-        if not content and not message.attachments:
+        if not user_text and not message.attachments:
             await message.reply("Hey! How can I help?")
             return
 
+        # Build caller-identity header so the agent always knows who is asking.
+        author = message.author
+        display = getattr(author, "display_name", author.name)
+        guild_name = message.guild.name if message.guild else "DM"
+        channel_name = getattr(message.channel, "name", "unknown")
+        discord_ctx = (
+            "[DISCORD_CONTEXT]\n"
+            f"User: @{author.name}"
+            + (f" (display: {display})" if display != author.name else "")
+            + f" | ID: {author.id}\n"
+            f"Channel: #{channel_name} | Guild: {guild_name}\n"
+            "[/DISCORD_CONTEXT]\n\n"
+        )
+        content = (discord_ctx + user_text).strip()
+
         if isinstance(message.channel, discord.Thread):
             thread = message.channel
         else:
-            name = self._thread_name(content) if content else "Attachment"
+            name = self._thread_name(user_text) if user_text else "Attachment"
             thread = await message.create_thread(
                 name=name,
                 auto_archive_duration=1440,