dev: start-* launchers always start from a clean setup

The launchers reused whatever was left under $XDG_CACHE_HOME/nuwiki-dev
between runs — seed_wiki() skipped reseeding when index.wiki already
existed, so stale edits from a previous session leaked into the next, and
old swap/viminfo/coc state carried over.

Each launch now starts clean:
- seed_wiki() reseeds the managed scratch wiki from a pristine copy every
  run (rm -rf + write_sample_wiki). A user-pointed NUWIKI_DEV_WIKI is never
  wiped — it's seeded only when empty, so a real wiki is never clobbered.
  NUWIKI_DEV_NO_SEED still bypasses seeding entirely.
- new reset_dirs() helper wipes + recreates each launcher's owned state dirs
  (vim viminfo/swap/undo, coc data, nvim XDG state/data/config) before launch.

Cached plugin clones (vim-lsp, async.vim, coc.nvim) are kept — only the
wiki and editor state are reset. Header docs updated to describe the
fresh-each-launch behavior.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

development/start-vim-coc.sh

@@ -29,6 +29,11 @@
 # the running LSP. `cargo build --release` is incremental, so this is a fast
 # no-op when nothing changed. Set NUWIKI_DEV_SKIP_BUILD=1 to skip the build.
 #
+# Each launch starts clean: the scratch wiki is reseeded from a pristine
+# copy and the editor state (viminfo/swap/undo, coc data) is wiped, so stale
+# edits never carry over. The cached coc.nvim clone is kept. A wiki you point
+# at with NUWIKI_DEV_WIKI is never wiped (seeded only if empty).
+#
 # Tested with Vim 9.1+ and Node 18+. State (viminfo/sessions, coc data) lives
 # under $XDG_CACHE_HOME/nuwiki-dev/ so your real Vim install stays untouched.
 
@@ -165,6 +170,7 @@ main() {
   write_coc_settings
   seed_wiki
   write_vimrc
+  reset_dirs "$VIM_STATE" "$COC_DATA"
   mkdir -p "$VIM_STATE/swap" "$VIM_STATE/backup" "$VIM_STATE/undo" "$COC_DATA"
 
   log "launching: vim --clean -u $VIMRC"