add persistent chat history with cross-device sync

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+# Chat History Persistence & Cross-Device Sync
+
+## New File: `src/chat-history.ts`
+- Added `PersistedMessage` and `ChatHistoryData` interfaces for typed storage
+- `loadChatHistory()` — reads from `chat-history.json` in the plugin folder via `vault.adapter`
+- `saveChatHistory()` — writes user/assistant messages to disk (strips system/tool messages)
+- `clearChatHistory()` — writes an empty history file (not deletion) for reliable Obsidian Sync propagation
+- `toPersistableMessages()` / `toRuntimeMessages()` — conversion between runtime `ChatMessage[]` and storage format
+- `isValidChatHistory()` — strict type guard for safe JSON parsing with version check
+
+## Modified: `src/chat-view.ts`
+- On **open**: restores persisted chat history, renders user messages as plain text and assistant messages as markdown (with wiki-link navigation)
+- On **message send**: debounced save (500ms) after user message and after assistant response completes
+- On **close**: flushes pending save and cleans up debounce timer
+- On **clear chat**: writes empty history file and updates sync snapshot
+- Added `reloadChatHistory()` public method for external sync triggers; skips reload if streaming is active to avoid UI disruption
+- Added `saveChatHistoryDebounced()` with snapshot update to prevent false sync reloads from local writes
+
+## Modified: `src/main.ts`
+- Added `onExternalSettingsChange()` — reloads settings and checks for chat history changes when Obsidian Sync updates `data.json`
+- Added `visibilitychange` DOM event listener — checks for synced changes when the app regains focus (covers device switching)
+- Added `checkChatHistorySync()` — snapshot-based change detection that reloads the chat view only when the persisted file differs from the known state
+- Added `updateChatSnapshot()` — called after local saves and restores to prevent false sync triggers
+- Added `buildChatSnapshot()` helper — lightweight string comparison using message count and last message content