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.

CONTRIBUTING.md

@@ -7,6 +7,67 @@ open pull requests there. It's approaching its first public release, so
 the most useful contributions right now are bug reports and rough edges
 you hit while learning.
 
+## Getting set up
+
+The toolchain is pinned with a Nix flake, so dev and CI share one Rust
+version. With [Nix](https://nixos.org/download) (flakes enabled):
+
+```sh
+nix develop          # a shell with the pinned toolchain
+```
+
+Run everything through `nix develop -c …` so you match CI exactly.
+
+## The checks your change must pass
+
+CI runs these on every push and PR; run them locally first, and **check the
+exit code** (a piped `… | tail` can hide a failure):
+
+```sh
+nix develop -c cargo fmt --check
+nix develop -c cargo clippy --all-targets -- -D warnings
+nix develop -c cargo test
+```
+
+All three must be green, with no skipped tests. New behaviour needs tests —
+the suite runs in tiers from unit up to a PTY-driven end-to-end harness.
+
+## Branches and pull requests
+
+- **`main` is protected** and always green; it isn't pushed to directly.
+  Every change lands through a pull request.
+- **Branch off `main`**, one logical change per branch, named with a
+  conventional prefix: `feat/…`, `fix/…`, `docs/…`, `chore/…`,
+  `refactor/…`, `test/…`, `ci/…`.
+- **Commit messages** follow [Conventional Commits](https://www.conventionalcommits.org/)
+  (`feat:`, `fix:`, `docs:` …) and reference the issue (`… (#123)` /
+  `Closes #123`).
+- **Open a PR against `main`.** The CI gate must pass before it can merge;
+  PRs land as merge commits — history is append-only, so please don't
+  force-push or rebase shared branches.
+- Keep one PR to one concern; don't fold unrelated changes together.
+
+## User-facing text
+
+Two rules bind anything a user can see (errors, help, notes):
+
+- **Don't name the database engine** — say "the database" / "the engine".
+- **Don't say "DSL"** — say "simple mode" / "advanced mode".
+
+## Significant changes get a decision record
+
+Architectural or otherwise consequential changes are recorded as ADRs in
+[`docs/adr/`](docs/adr/) — start with [`docs/adr/README.md`](docs/adr/README.md).
+If your change touches a decided area, read the relevant ADR first; if it
+would change a decision, propose a new ADR rather than quietly diverging.
+Opening an issue to discuss a substantial change before building it is
+always welcome.
+
+## Code style
+
+Match the surrounding code — its naming, comment density, and idioms. The
+Clippy nursery lints are enabled and must pass clean.
+
 ## License of contributions
 
 Unless you explicitly state otherwise, any contribution you intentionally