This PR introduces explicit, user-declared message priority to the message queue

What changed

1. Explicit message priority (opt-in)

Users can now declare priority directly in the message text using a structured prefix:

[priority: high] {prompt …}
[priority: normal] {prompt …}
[priority: low] {prompt …}

• Priority is parsed deterministically from the first non-empty text part.
• The prefix is removed before execution and does not reach the model.
• Messages without an explicit prefix default to normal.

Priority is stored as metadata on each queued message.

2. Priority-aware queue draining

Queue draining now selects the next message by:

1. Highest priority
2. FIFO order within the same priority

3. Correct hold-mode semantics

In hold mode, all messages are now queued, including the first message in an idle session.

This ensures that:

• priority extraction always runs,
• queue state is consistent from the first message onward,
• and hold behaves as an explicit, predictable buffering mode.

immediate mode behavior is unchanged.

Backwards compatibility

• Default behavior remains unchanged unless hold mode is explicitly enabled.
• Messages without a priority tag behave exactly as before (normal priority).
• No changes to /queue command syntax.

Testing

Manually tested:

• hold mode with first message queued
• Mixed priority messages (high, normal, low)
• FIFO ordering within same priority
• Prefix stripping before execution
• immediate mode unaffected