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/0000-record-architecture-decisions.md

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+# ADR-0000: Record architecture decisions
+
+## Status
+
+Accepted
+
+## Context
+
+The RDBMS Playground project will accumulate design decisions as it
+grows. We want those decisions to be traceable, reviewable, and easy
+to challenge later when context has changed.
+
+## Decision
+
+Record significant architecture and product decisions as Architecture
+Decision Records (ADRs) in `docs/adr/`, using the Michael Nygard
+format (Status, Context, Decision, Consequences). Files are numbered
+sequentially with a four-digit prefix and a kebab-case slug, e.g.
+`0001-language-and-tui-framework.md`.
+
+Each ADR captures one decision. Superseding a decision means adding a
+new ADR that references the old one and marking the old one as
+"Superseded by ADR-NNNN".
+
+ADRs document decisions, not speculation. An idea under discussion
+belongs in conversation, an issue, or a design note — not an ADR. An
+ADR is written once a decision has actually been made.
+
+## Index discipline
+
+`docs/adr/README.md` contains the canonical index of all ADRs.
+Whenever an ADR is added, renamed, or has its status changed (e.g.
+"Superseded by ADR-NNNN"), the index MUST be updated in the same
+change. An ADR change without a corresponding index update is
+incomplete.
+
+The index lists ADRs in numerical order. Each entry shows the
+number, title, and — where relevant — status annotations such as
+"Superseded by ADR-NNNN" or "Deprecated".
+
+## Consequences
+
+- New significant decisions require an ADR before or alongside the
+  implementation that depends on them.
+- Old decisions remain visible even after they are superseded.
+- Reviewers can audit the rationale chain by reading `docs/adr/` in
+  order.
+- The index in `README.md` stays trustworthy because keeping it
+  current is part of every ADR change, not an afterthought.