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/test-wt.sh

@@ -0,0 +1,57 @@
+#!/usr/bin/env bash
+# Smoke-tests wt-new.sh / wt-clean.sh against a local file:// origin.
+set -euo pipefail
+
+here="$(cd "$(dirname "$0")" && pwd)"
+WT_NEW="$here/wt-new.sh"
+WT_CLEAN="$here/wt-clean.sh"
+PASS=0 FAIL=0
+check() { if [ "$2" = "$3" ]; then echo "PASS: $1 ($3)"; PASS=$((PASS+1));
+          else echo "FAIL: $1 — expected '$2' got '$3'"; FAIL=$((FAIL+1)); fi; }
+
+T="$(mktemp -d)"
+trap 'rm -rf "$T"' EXIT
+
+git init -q --bare "$T/origin.git"
+git clone -q "$T/origin.git" "$T/primary"
+git -C "$T/primary" config user.email t@t.io
+git -C "$T/primary" config user.name tester
+echo seed > "$T/primary/README.md"
+git -C "$T/primary" add -A
+git -C "$T/primary" commit -qm seed
+git -C "$T/primary" branch -M main
+git -C "$T/primary" push -q origin main
+
+run() { ( cd "$T/primary" && "$@" ); }
+target="$T/primary-worktree-thing-one"
+
+echo "--- wt-new rejects a non-conventional branch name ---"
+set +e; run bash "$WT_NEW" my-thing >/dev/null 2>&1; rc=$?; set -e
+check "bad-prefix rejected" "yes" "$( [ "$rc" -ne 0 ] && echo yes || echo no )"
+
+echo "--- wt-new creates branch + sibling worktree ---"
+out="$(run bash "$WT_NEW" feat/thing-one 2>/dev/null)"
+check "prints target path" "$target" "$out"
+check "worktree dir exists" "yes" "$( [ -d "$target" ] && echo yes || echo no )"
+check "worktree on the new branch" "feat/thing-one" "$(git -C "$target" rev-parse --abbrev-ref HEAD)"
+
+echo "--- wt-new refuses to clobber an existing branch ---"
+set +e; run bash "$WT_NEW" feat/thing-one >/dev/null 2>&1; rc=$?; set -e
+check "duplicate rejected" "yes" "$( [ "$rc" -ne 0 ] && echo yes || echo no )"
+
+echo "--- wt-clean removes a MERGED worktree (and keeps an unmerged one) ---"
+# thing-one: make it merged into origin/main (push its tip to main).
+git -C "$target" commit -q --allow-empty -m "thing-one work"
+git -C "$target" push -q origin HEAD:main
+# thing-two: a second worktree that is NOT merged.
+run bash "$WT_NEW" feat/thing-two >/dev/null 2>&1
+target2="$T/primary-worktree-thing-two"
+git -C "$target2" commit -q --allow-empty -m "thing-two work (unmerged)"
+
+run bash "$WT_CLEAN" >/dev/null 2>&1
+check "merged worktree removed" "no"  "$( [ -d "$target"  ] && echo yes || echo no )"
+check "unmerged worktree kept"  "yes" "$( [ -d "$target2" ] && echo yes || echo no )"
+
+echo
+echo "==== $PASS passed, $FAIL failed ===="
+[ "$FAIL" -eq 0 ]