D7: editor multi-cursor support

Add secondary_cursors field to Editor with insert_char_multi,
delete_back_multi, delete_forward_multi methods. Right-to-left
processing ensures position shifts don't corrupt earlier insertions.

7 new tests: add/clear, all_positions, insert, delete_back,
delete_forward, unicode, duplicate-add.

AGENTS.md

@@ -746,7 +746,7 @@ All custom work goes in `local/` — see `local/AGENTS.md` for fork model usage.
 - Build requires Linux x86_64 host, 8GB+ RAM, 20GB+ disk
 - QEMU used for testing (make qemu). VirtualBox also supported
 - The `repo` binary (cookbook CLI) may crash with TUI in non-interactive environments — use `CI=1`
-- No git submodules — external repos managed via recipe source URLs and repo manifests
+- 9 git submodules (core component forks on `submodule/<component>` branches). NO new branches and NO new submodules — see `local/AGENTS.md` § BRANCH AND SUBMODULE POLICY
 - Historical integration report removed (2026-04-16); see `local/docs/CONSOLE-TO-KDE-DESKTOP-PLAN.md` for current state
 
 ### Stale Prefix Detection