docs: Clarify redirect behavior in API routes and store migration requirement

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: KATO-Hiro

.claude/rules/coding-style.md

@@ -22,7 +22,7 @@ Common identifiers: `typescript`, `svelte`, `sql`, `bash`, `mermaid`, `json`, `p
 ## SvelteKit: Routes vs API Endpoints
 
 - Page routes (`+page.server.ts`): use `redirect()` to navigate
-- API routes (`+server.ts`): use `error()` — throwing `redirect()` causes `fetch` clients to receive HTML instead of a JSON error
+- API routes (`+server.ts`): use `error()` — throwing `redirect()` returns a 3xx response; `fetch` follows it by default and receives the HTML page at the redirect target instead of a JSON error
 
 ## Dual-Enforcement Constraints
 

.claude/rules/svelte-components.md

@@ -25,7 +25,7 @@ Use `$props()`, `$state()`, `$derived()`, `$effect()` in all components. Props p
 ## File Naming
 
 - Components: `PascalCase.svelte`
-- Stores: `snake_case.svelte.ts` in `src/lib/stores/`, class-based with `$state()`, export singleton
+- Stores: `snake_case.svelte.ts` in `src/lib/stores/`, class-based with `$state()`, export singleton. Pre-Runes stores (using `writable()`, `.ts` extension) must be migrated to this pattern before adding features or extending them.
 
 ## Flowbite Svelte