feat(features): add hook message injector

- Port hook-message-injector from opencode-cc-plugin (4 files)
- constants.ts: XDG-based path definitions (MESSAGE_STORAGE, PART_STORAGE)
- types.ts: MessageMeta, OriginalMessageContext, TextPart interfaces
- injector.ts: injectHookMessage() implementation with message/part storage
- index.ts: Barrel export
- Self-contained module with no import path changes needed
- Preserves XDG_DATA_HOME environment variable support
- Preserves message fallback logic for incomplete originalMessage

🤖 GENERATED WITH ASSISTANCE OF [OhMyOpenCode](https://github.com/code-yeongyu/oh-my-opencode)

src/features/hook-message-injector/constants.ts

@@ -0,0 +1,8 @@
+import { join } from "node:path"
+import { homedir } from "node:os"
+
+const xdgData = process.env.XDG_DATA_HOME || join(homedir(), ".local", "share")
+
+export const OPENCODE_STORAGE = join(xdgData, "opencode", "storage")
+export const MESSAGE_STORAGE = join(OPENCODE_STORAGE, "message")
+export const PART_STORAGE = join(OPENCODE_STORAGE, "part")