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/_common.sh

@@ -12,7 +12,10 @@
 #   log                             — prefixed status line
 #   ensure_binary                   — build + symlink nuwiki-ls
 #   write_sample_wiki DIR           — lay down the feature-showcase wiki
-#   seed_wiki                       — seed WIKI_DIR (respects NUWIKI_DEV_NO_SEED)
+#   reset_dirs DIR...               — wipe + recreate owned state dirs
+#   seed_wiki                       — (re)seed WIKI_DIR fresh each run; never
+#                                     touches a user-supplied NUWIKI_DEV_WIKI;
+#                                     respects NUWIKI_DEV_NO_SEED
 
 # Resolve paths relative to this file so it works regardless of which
 # launcher sources it.
@@ -261,16 +264,37 @@ A diary entry. Back to [[diary]].
 EOF
 }
 
+# reset_dirs DIR... — wipe and recreate each dir so a launch always starts
+# from clean editor state (swap/undo/viminfo, coc data, nvim XDG dirs). Only
+# ever called on dirs the launchers own under $DEV_DIR.
+reset_dirs() {
+  local d
+  for d in "$@"; do
+    rm -rf "$d"
+    mkdir -p "$d"
+  done
+}
+
 seed_wiki() {
   if [[ "${NUWIKI_DEV_NO_SEED:-0}" == "1" ]]; then
     log "NUWIKI_DEV_NO_SEED=1 — skipping wiki seed (using $WIKI_DIR as-is)"
     return
   fi
-  mkdir -p "$WIKI_DIR" "$WIKI_DIR/diary"
-  # Seed the whole showcase as a unit; skip when an index already exists
-  # so a re-run never clobbers edits made while testing.
-  if [[ -f "$WIKI_DIR/index.wiki" ]]; then
+  # A user-pointed wiki (NUWIKI_DEV_WIKI) is never wiped — it might be a real
+  # wiki. Seed the sample only when it has no index yet, so we don't clobber it.
+  if [[ -n "${NUWIKI_DEV_WIKI:-}" ]]; then
+    mkdir -p "$WIKI_DIR" "$WIKI_DIR/diary"
+    if [[ -f "$WIKI_DIR/index.wiki" ]]; then
+      log "using existing wiki at $WIKI_DIR (left untouched)"
+      return
+    fi
+    write_sample_wiki "$WIKI_DIR"
     return
   fi
+  # Managed scratch wiki under the dev cache: always start from a pristine
+  # copy so stale edits from a previous run never leak into the next.
+  log "reseeding scratch wiki (fresh copy): $WIKI_DIR"
+  rm -rf "$WIKI_DIR"
+  mkdir -p "$WIKI_DIR" "$WIKI_DIR/diary"
   write_sample_wiki "$WIKI_DIR"
 }