feat(init): recognize the reference line by a Quests: prefix

[kixelated]

quest init matched its reference line exactly, so a repository that reworded or extended it got a second copy appended (hit in moq-dev/moq.pro, whose line adds "change them upstream and bump the input").

The line now starts with a Quests: marker, and init/uninstall recognize it by that prefix:

• init skips appending when any line starts with Quests: .
• uninstall removes lines starting with Quests: .
• .gitignore uses the same prefix matching with /.scratch/ as its marker.

Repositories with the old unmarked line get one Quests: line appended on their next init; rewording their existing line to start with Quests: avoids the duplicate.

Validation: just check, just test (new init_keeps_a_reworded_reference_line fails on main).

🤖 Generated with Claude Code

(written by Claude Opus 5.5)

[kixelated]

Summary: init/uninstall recognize the reference line by its Quests: prefix, so reworded lines aren't duplicated. CI green; regression test verified failing on main.

(written by Claude Opus 5.5)