v0.7.2 — Flatten tool schemas

Flatten both edit and hashline_edit tool schemas per lessons from Armin Ronacher's Better Models: Worse Tools.

hashline_edit: removed hashlineChanges[] nested array. Now uses flat start_hash, end_hash, replacement strings.

edit: added start_hash, end_hash, replacement (flat hashline path) and oldText, newText (flat legacy path). edits[] no longer required.

E_BOUNDARY_DUP: promoted from warning to hard error. Boundary duplicates rejected with actionable message.

4 files changed, 75 insertions, 76 deletions.