fix(commands): intercept non-priority commands during active turn

Non-priority slash commands (e.g. /new, /help, /dream-log) arriving
while a session has an active LLM turn were silently queued into the
pending injection buffer and later injected as raw user messages into
the LLM conversation. This caused the model to respond to "/new" as
plain text instead of executing the command.

Root cause: the run() loop only checked priority commands (/stop,
/restart, /status) before routing messages to the pending queue. All
other command tiers (exact, prefix) bypassed command dispatch entirely.

Changes:
- Add CommandRouter.is_dispatchable_command() to match exact/prefix
  tiers, mirroring the existing is_priority() pattern.
- In run(), intercept dispatchable commands before pending queue
  insertion and dispatch them directly via _dispatch_command_inline().
- Extract _cancel_active_tasks() from cmd_stop for reuse; cmd_new now
  cancels active tasks before clearing the session to prevent shared
  mutable state corruption from concurrent asyncio coroutines.
- Update /new semantics: stops active task first, then clears session.
- Update documentation in help text, docs, and Discord command list.

docs/chat-commands.md

@@ -4,7 +4,7 @@ These commands work inside chat channels and interactive agent sessions:
 
 | Command | Description |
 |---------|-------------|
-| `/new` | Start a new conversation |
+| `/new` | Stop current task and start a new conversation |
 | `/stop` | Stop the current task |
 | `/restart` | Restart the bot |
 | `/status` | Show bot status |