Initial planning docs: CLAUDE.md and ADRs 0000-0008

Captures up-front design decisions for RDBMS Playground:
stack (Rust + Ratatui + SQLite), input modes, project file
format, type vocabulary, undo snapshots and replay log,
sharing/export, and testing approach. ADR-0000 establishes
the ADR practice itself and mandates index upkeep alongside
any ADR change.

docs/adr/0007-sharing-and-export.md

@@ -0,0 +1,59 @@
+# ADR-0007: Sharing and export
+
+## Status
+
+Accepted
+
+## Context
+
+Learners and instructors will want to share projects: an
+instructor distributing an exercise, a student attaching a
+project to a question, a collaborator sending a snapshot.
+
+The need is real, but committing to a hosted "publish" feature
+(account system, server, namespace, moderation) would dwarf the
+core app and is not warranted for a teaching tool. We can satisfy
+the need entirely with local artifacts and documentation.
+
+## Decision
+
+No hosted publishing feature. Sharing is supported via two
+mechanisms:
+
+1. **`export` command.**
+   - Available as an app-level command in both input modes
+     (ADR-0003).
+   - Produces a single zip file containing `project.yaml`,
+     `data/`, and `history.log`, **excluding** `playground.db`
+     (the recipient rebuilds it on open per ADR-0004).
+   - Default output path is the parent directory of the project.
+   - Default filename:
+     `YYYYMMDD-<projectname>-export-<sequence>.zip`, where
+     `YYYYMMDD` is today's local date and `<sequence>` is a
+     two-digit zero-padded counter that starts at `01` and is
+     incremented to avoid clobbering an existing file in the
+     output directory on the same day.
+   - An explicit path and/or filename may be given to override
+     the default.
+
+2. **Documented recipes.**
+   - Each new project includes a `.gitignore` template that
+     excludes `playground.db`, so committing a project to git
+     yields a clean, diff-friendly history.
+   - The user-facing documentation includes recipes for sharing
+     via git, email, and direct file transfer.
+
+If real-world usage later reveals friction these mechanisms cannot
+solve, a publish feature can be revisited as a separate ADR.
+
+## Consequences
+
+- Zero server-side surface area, zero accounts, zero hosting
+  costs.
+- Sharing is a power-user-friendly workflow from day one without
+  blocking on infrastructure decisions.
+- Recipients reconstruct the database deterministically from the
+  authoritative text sources — sharing is automatically a
+  validation of the rebuild path.
+- If a future hosted feature is ever added, this ADR is
+  superseded, not extended.