compile --mode compiled carries the 1.9.3 template's ## Environment placeholder into the retained block #364

Found during 2.0.0 release prep (#361), on the "upgrade from 1.9.3" path.

A CLAUDE.md written by 1.9.3 contains the template's ## Environment placeholder (with its HTML comment). If a user also hand-added a second ## Environment section, then charter adf compile --target claude --write --mode compiled (#358) carries both into the retained block, placeholder comment included.

Expected: the untouched template placeholder is recognised as template text (like linesRestoreWouldDrop's template exemption in #353) and not carried. The user's own section is carried as before.

Repro: npx -y @stackbilt/cli@1.9.3 bootstrap --yes, append a second ## Environment with one line to CLAUDE.md, then run the 2.0 build's adf compile --target claude --write --mode compiled and inspect the retained block.