chore(workflow): branch-and-PR working method + ADR-number reservation (ADR-0059)

Adopt a trackable working method now the repo is public:
- PRs onto a protected main; --no-ff merge commits; one worktree per branch.
- Reserve-first ADR numbering: scripts/adr-reserve.sh claims the next number
  atomically against main (push = compare-and-swap; ledger
  docs/adr/RESERVATIONS.log), so a number is stable from creation.
- Worktree helpers scripts/wt-new.sh + wt-clean.sh.
- Local-origin test harnesses (reserve 10/10, worktrees 7/7, shellcheck clean).

Record the decision in ADR-0059, supersede ADR-0000's placeholder-until-merge
numbering default, and add the lean CONTRIBUTING.md sections + CLAUDE.md
operational rules.

scripts/wt-clean.sh

@@ -0,0 +1,72 @@
+#!/usr/bin/env bash
+#
+# wt-clean.sh — tidy up worktrees whose branch has merged.
+#
+# WHAT it does (safe by default):
+#   1. `git worktree prune` — drop admin entries for worktrees whose
+#      directory is already gone.
+#   2. For every *other* worktree (never the primary checkout, never the one
+#      you're standing in) whose branch is fully merged into origin/main:
+#      remove the worktree and delete the local branch.
+#
+# SAFETY: it refuses to remove a worktree with uncommitted changes (no
+# --force) and only ever touches branches git confirms are merged into
+# origin/main, so unmerged work is never lost. Run it whenever; it's not
+# tied to any moment.
+#
+# USAGE
+#   scripts/wt-clean.sh            # do it
+#   scripts/wt-clean.sh --dry-run  # show what it would remove
+
+set -euo pipefail
+
+log() { printf '  wt-clean: %s\n' "$*" >&2; }
+
+dry=0
+[ "${1:-}" = "--dry-run" ] && dry=1
+
+git rev-parse --git-dir >/dev/null 2>&1 || { echo "wt-clean: not in a git repo" >&2; exit 1; }
+
+log "fetching origin/main"
+git fetch --quiet --prune origin main || true
+
+log "pruning stale worktree entries"
+if (( dry )); then
+  git worktree prune --dry-run -v
+else
+  git worktree prune -v
+fi
+
+# The primary checkout and the current worktree are off-limits.
+main_wt="$(git worktree list --porcelain | awk '/^worktree /{print $2; exit}')"
+here="$(git rev-parse --show-toplevel)"
+
+# Branches fully merged into origin/main (whitespace-trimmed, sans markers).
+merged="$(git branch --format='%(refname:short)' --merged origin/main)"
+is_merged() { grep -qxF "$1" <<<"$merged"; }
+
+removed=0
+# Iterate worktrees: paired "worktree <path>" / "branch refs/heads/<name>".
+path=""; while read -r key val; do
+  case "$key" in
+    worktree) path="$val" ;;
+    branch)
+      br="${val#refs/heads/}"
+      if [ "$path" != "$main_wt" ] && [ "$path" != "$here" ] && is_merged "$br"; then
+        if (( dry )); then
+          log "would remove worktree $path (branch $br, merged)"
+        else
+          if git worktree remove "$path" 2>/dev/null; then
+            git branch -d "$br" >/dev/null 2>&1 || true
+            log "removed worktree $path + branch $br"
+            removed=$((removed+1))
+          else
+            log "skipped $path — has uncommitted changes (remove it by hand)"
+          fi
+        fi
+      fi
+      ;;
+  esac
+done < <(git worktree list --porcelain)
+
+(( dry )) || log "done — removed $removed merged worktree(s)"