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/config.py

@@ -22,6 +22,14 @@ class Settings(BaseSettings):
         description="Path to ClaudeTools repository (agent cwd)",
     )
 
+    # Path to Discord-specific system prompt, relative to claudetools_root.
+    # Edit projects/discord-bot/DISCORD_CLAUDE.md to change bot behavior without
+    # touching the main CLAUDE.md. Changes take effect on next bot restart.
+    discord_system_prompt: Path = Field(
+        default=Path("projects/discord-bot/DISCORD_CLAUDE.md"),
+        description="System prompt for Discord bot sessions (relative to claudetools_root)",
+    )
+
     log_level: str = Field(default="INFO", description="Logging level")
     log_file: Optional[Path] = Field(default=Path("logs/bot.log"), description="Log file")