Slim CLAUDE.md with progressive disclosure, add project purpose

Move niche conventions (component props, export compat) to
docs/conventions.md, trim Speckit/Constitution sections to link to
source files, and add a one-line project description.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs/conventions.md

@@ -0,0 +1,20 @@
+# Conventions (detailed)
+
+These conventions supplement the overview in `CLAUDE.md`. Load this file when working in the relevant areas.
+
+## Component Props
+
+Max 8 explicitly declared props per component interface, enforced by `scripts/check-component-props.mjs` (uses the TypeScript compiler API). Run `pnpm check:props` to verify.
+
+- Use React context for shared state
+- Reserve props for per-instance config (data items, layout variants, refs)
+
+## Export Format Compatibility
+
+When changing `Encounter`, `Combatant`, `PlayerCharacter`, or `UndoRedoState` types, verify that previously exported JSON files (version 1) still import correctly. If not, bump the `ExportBundle` version and add migration logic in `validateImportBundle()`.
+
+## Domain Patterns
+
+- **Branded types** for identity values (e.g., `CombatantId`). Prefer immutability/`readonly` where practical. See [ADR-003](adr/003-branded-types-for-identity.md).
+- **Domain events** are plain data objects with a `type` discriminant — no classes. See [ADR-002](adr/002-domain-events-as-plain-data.md).
+- **Errors as values** (`DomainError`), never thrown. See [ADR-001](adr/001-errors-as-values.md).