feat(shared): add plugin identity constants for package name migration

Add centralized plugin identity constants to support migration from
oh-my-opencode to oh-my-openagent. Includes both current and legacy
names for backward compatibility.

🤖 Generated with assistance of OhMyOpenCode

src/shared/plugin-identity.test.ts

@@ -0,0 +1,89 @@
+import { describe, it, expect } from "bun:test"
+import {
+  PLUGIN_NAME,
+  LEGACY_PLUGIN_NAME,
+  CONFIG_BASENAME,
+  LEGACY_CONFIG_BASENAME,
+  LOG_FILENAME,
+  CACHE_DIR_NAME,
+  LEGACY_CACHE_DIR_NAME,
+} from "./plugin-identity"
+
+describe("plugin-identity constants", () => {
+  describe("PLUGIN_NAME", () => {
+    it("equals oh-my-openagent", () => {
+      // given
+
+      // when
+
+      // then
+      expect(PLUGIN_NAME).toBe("oh-my-openagent")
+    })
+  })
+
+  describe("LEGACY_PLUGIN_NAME", () => {
+    it("equals oh-my-opencode", () => {
+      // given
+
+      // when
+
+      // then
+      expect(LEGACY_PLUGIN_NAME).toBe("oh-my-opencode")
+    })
+  })
+
+  describe("CONFIG_BASENAME", () => {
+    it("equals oh-my-openagent", () => {
+      // given
+
+      // when
+
+      // then
+      expect(CONFIG_BASENAME).toBe("oh-my-openagent")
+    })
+  })
+
+  describe("LEGACY_CONFIG_BASENAME", () => {
+    it("equals oh-my-opencode", () => {
+      // given
+
+      // when
+
+      // then
+      expect(LEGACY_CONFIG_BASENAME).toBe("oh-my-opencode")
+    })
+  })
+
+  describe("LOG_FILENAME", () => {
+    it("equals oh-my-openagent.log", () => {
+      // given
+
+      // when
+
+      // then
+      expect(LOG_FILENAME).toBe("oh-my-openagent.log")
+    })
+  })
+
+  describe("CACHE_DIR_NAME", () => {
+    it("equals oh-my-openagent", () => {
+      // given
+
+      // when
+
+      // then
+      expect(CACHE_DIR_NAME).toBe("oh-my-openagent")
+    })
+  })
+
+  describe("LEGACY_CACHE_DIR_NAME", () => {
+    it("equals oh-my-opencode", () => {
+      // given
+
+      // when
+
+      // then
+      expect(LEGACY_CACHE_DIR_NAME).toBe("oh-my-opencode")
+    })
+  })
+})