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-new.sh

@@ -0,0 +1,54 @@
+#!/usr/bin/env bash
+#
+# wt-new.sh — start a branch in its own git worktree.
+#
+# WHY: every branch is developed in an isolated worktree, never by switching
+# the primary checkout (two shells in one checkout is how you clobber work).
+# This creates the branch off an up-to-date `main` and a sibling worktree
+# named to match the repo's existing convention:
+#
+#     <parent>/<repo>-worktree-<last-segment-of-branch>
+#     e.g.  feat/clause-concept-hints  ->  ../rdbms-playground-worktree-clause-concept-hints
+#
+# USAGE
+#   scripts/wt-new.sh <branch>
+#   scripts/wt-new.sh feat/clause-concept-hints
+#
+# Prints the new worktree path; cd there to start working.
+
+set -euo pipefail
+
+log() { printf '  wt-new: %s\n' "$*" >&2; }
+die() { printf 'wt-new: ERROR: %s\n' "$*" >&2; exit 1; }
+
+[ $# -eq 1 ] || die "usage: wt-new.sh <branch>  (e.g. feat/my-thing)"
+branch="$1"
+
+# Conventional prefixes keep branch names aligned with commit types.
+case "$branch" in
+  feat/*|fix/*|docs/*|chore/*|refactor/*|test/*|ci/*) : ;;
+  *) die "branch should start with feat/ fix/ docs/ chore/ refactor/ test/ ci/ (got: $branch)" ;;
+esac
+
+git rev-parse --git-dir >/dev/null 2>&1 || die "not inside a git repo"
+
+# Sibling-of-the-primary-checkout location, matching existing worktrees.
+main_wt="$(git worktree list --porcelain | awk '/^worktree /{print $2; exit}')"
+parent="$(dirname "$main_wt")"
+repo="$(basename "$main_wt")"
+seg="${branch##*/}"
+target="$parent/${repo}-worktree-${seg}"
+
+[ -e "$target" ] && die "worktree path already exists: $target"
+git show-ref --verify --quiet "refs/heads/$branch" \
+  && die "branch already exists: $branch (use 'git worktree add' to attach a worktree to it)"
+
+log "fetching origin/main"
+git fetch --quiet origin main || die "git fetch origin main failed"
+
+log "creating branch '$branch' + worktree at $target"
+git worktree add --quiet -b "$branch" "$target" origin/main \
+  || die "git worktree add failed"
+
+log "ready — cd into it:"
+echo "$target"