docs: add .memory consultation + instruction file sync rules

Add two new sections to AGENTS.md and CLAUDE.md:
- .memory Files: consult knowledge.md, decisions.md, plans/, research/ at task start
- Instruction File Sync: keep AGENTS.md, CLAUDE.md, .cursorrules, and Copilot CLI instructions in sync when any is updated

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

AGENTS.md

@@ -7,7 +7,7 @@
 
 ## Architecture Overview
 - **API proxy pattern**: Frontend never calls Django directly. All API calls go through `frontend/src/routes/api/[...path]/+server.ts`, which proxies to `http://server:8000`, handles cookies, and injects CSRF behavior.
-- **AI chat**: Embedded in Collections → Recommendations via `AITravelChat.svelte` component. No standalone `/chat` route. Provider list is dynamic from backend `GET /api/chat/providers/` (sourced from LiteLLM runtime + custom entries like `opencode_zen`). Chat conversations use SSE streaming via `/api/chat/conversations/`.
+- **AI chat**: Embedded in Collections → Recommendations via `AITravelChat.svelte` component. No standalone `/chat` route. Provider list is dynamic from backend `GET /api/chat/providers/` (sourced from LiteLLM runtime + custom entries like `opencode_zen`). Chat conversations use SSE streaming via `/api/chat/conversations/`. Chat composer supports per-provider model override (persisted in browser `localStorage`). LiteLLM errors are mapped to sanitized user-safe messages via `_safe_error_payload()` (never exposes raw exception text).
 - **Service ports**:
   - `web` → `:8015`
   - `server` → `:8016`
@@ -50,8 +50,8 @@ Run in this order:
 4. `cd frontend && bun run build`
 
 ## Known Issues (Expected)
-- Frontend `bun run check`: **3 type errors + 19 warnings** expected
-- Backend tests: **2/3 fail** (expected)
+- Frontend `bun run check`: **0 errors + 6 warnings** expected (pre-existing in `CollectionRecommendationView.svelte` + `RegionCard.svelte`)
+- Backend tests: **6/30 fail** (pre-existing: 2 user email key errors + 4 geocoding API mocks)
 - Docker dev setup has frontend-backend communication issues (500 errors beyond homepage)
 
 ## Key Patterns
@@ -60,8 +60,20 @@ Run in this order:
 - Styling: use DaisyUI semantic colors/classes (`bg-primary`, `text-base-content`, etc.)
 - Security: handle CSRF tokens via `/auth/csrf/` and `X-CSRFToken`
 - Chat providers: dynamic catalog from `GET /api/chat/providers/`; configured in `CHAT_PROVIDER_CONFIG`
+- Chat model override: composer text input for per-provider model selection; persisted in `localStorage` key `voyage_chat_model_prefs`; backend accepts optional `model` param in `send_message`
+- Chat error surfacing: `_safe_error_payload()` maps LiteLLM exceptions to sanitized user-safe categories (never forwards raw `exc.message`)
 
 ## Conventions
 - Do **not** attempt to fix known test/configuration issues as part of feature work.
 - Use `bun` for frontend commands, `uv` for local Python tooling where applicable.
 - Commit and merge completed feature branches promptly once validation passes (avoid leaving finished work unmerged).
+
+## .memory Files
+- At the start of any task, read `.memory/knowledge.md` and `.memory/decisions.md` for project context.
+- Check relevant files in `.memory/plans/` and `.memory/research/` for prior work on related topics.
+- These files capture architectural decisions, code review verdicts, security findings, and implementation plans from prior sessions.
+- Do **not** duplicate information from `.memory/` into code comments — keep `.memory/` as the single source of truth for project history.
+
+## Instruction File Sync
+- `AGENTS.md` (OpenCode), `CLAUDE.md` (Claude Code), `.cursorrules` (Cursor), and the Copilot CLI custom instructions must always be kept in sync.
+- Whenever any of these files is updated (new convention, new decision, new workflow rule), apply the equivalent change to all the others.