Add a "mid-build continuation" mode as a first-class flow

[FVDEDFOREVR]

Labels: enhancement

Body:
Currently the skill handles mid-build continuation as an edge case at the bottom of SKILL.md:

"Ask: 'What's been built so far, and what do you want to add or change?' Then treat the existing work as a constraint in the prompt."

This should be a proper first-class path — something the skill asks about upfront alongside depth/format/tone, or detects automatically when the user's phrasing suggests they're extending something that already exists ("add a sidebar to...", "update the nav to...", "extend this with...").

A good implementation would:

• Detect continuation intent from the trigger phrase
• Ask for a brief description of what exists
• Treat the existing code/design as a constraint block in the generated prompt
• Adjust the interview questions accordingly (skip "what are you building from scratch" type questions)