refactor(shared): unify MESSAGE_STORAGE/PART_STORAGE constants into single source

- Add src/shared/opencode-storage-paths.ts with consolidated constants

- Update imports in hook-message-injector and session-manager

- Add src/shared/opencode-storage-detection.ts with isSqliteBackend()

- Add OPENCODE_SQLITE_VERSION constant

- Export all from shared/index.ts

src/shared/opencode-storage-detection.ts

@@ -0,0 +1,23 @@
+import { existsSync } from "node:fs"
+import { join } from "node:path"
+import { getDataDir } from "./data-path"
+import { isOpenCodeVersionAtLeast, OPENCODE_SQLITE_VERSION } from "./opencode-version"
+
+let cachedResult: boolean | null = null
+
+export function isSqliteBackend(): boolean {
+  if (cachedResult !== null) {
+    return cachedResult
+  }
+  
+  const versionOk = isOpenCodeVersionAtLeast(OPENCODE_SQLITE_VERSION)
+  const dbPath = join(getDataDir(), "opencode", "opencode.db")
+  const dbExists = existsSync(dbPath)
+  
+  cachedResult = versionOk && dbExists
+  return cachedResult
+}
+
+export function resetSqliteBackendCache(): void {
+  cachedResult = null
+}