Keep user messages intact when automatic planning starts

[DevMando]

What changes

Desktop now routes high-confidence multi-step requests straight into the reviewable planner. It no longer changes the text of a user's message behind the scenes to persuade the model to call a planning tool.

Automatic planning is based on task shape: explicit checklists, cross-cutting changes, and multiple deliverables can trigger it. Questions, research, explanations, and narrow edits stay conversational. When Desktop starts a plan automatically, the transcript briefly explains why.

Rejected-plan follow-ups and forced skills now use separate, temporary system-role guidance. That guidance cannot be confused with user-authored text and does not carry into later turns.

Why users will notice

• Better plan decisions with fewer false positives
• No hidden pseudo-system text attached to user messages
• Clear reason shown when automatic planning begins
• /plan remains available as an explicit override

Also documented

The Unreleased changelog includes this behavior and the recently merged provider-reported token-total fix.

Validation

• 239/239 Desktop tests pass

This PR pins the matching engine branch commit until the engine PR is merged.