fix(#2823): auto-migrate legacy plugin name and warn users at startup

- logLegacyPluginStartupWarning now emits console.warn (visible to user,
  not just log file) when oh-my-opencode is detected in opencode.json
- Auto-migrates opencode.json plugin entry from oh-my-opencode to
  oh-my-openagent (with backup)
- plugin-config.ts: add console.warn when loading legacy config filename
- test: 10 tests covering migration, console output, edge cases

src/shared/migrate-legacy-plugin-entry.test.ts

@@ -0,0 +1,90 @@
+import { afterEach, beforeEach, describe, expect, it } from "bun:test"
+import { mkdirSync, readFileSync, rmSync, writeFileSync } from "node:fs"
+import { tmpdir } from "node:os"
+import { join } from "node:path"
+import { migrateLegacyPluginEntry } from "./migrate-legacy-plugin-entry"
+
+describe("migrateLegacyPluginEntry", () => {
+  let testDir = ""
+
+  beforeEach(() => {
+    testDir = join(tmpdir(), `omo-migrate-entry-${Date.now()}-${Math.random().toString(36).slice(2)}`)
+    mkdirSync(testDir, { recursive: true })
+  })
+
+  afterEach(() => {
+    rmSync(testDir, { recursive: true, force: true })
+  })
+
+  describe("#given opencode.json contains oh-my-opencode plugin entry", () => {
+    describe("#when migrating the config", () => {
+      it("#then replaces oh-my-opencode with oh-my-openagent", () => {
+        const configPath = join(testDir, "opencode.json")
+        writeFileSync(configPath, JSON.stringify({ plugin: ["oh-my-opencode@latest"] }, null, 2))
+
+        const result = migrateLegacyPluginEntry(configPath)
+
+        expect(result).toBe(true)
+        const content = readFileSync(configPath, "utf-8")
+        expect(content).toContain("oh-my-openagent@latest")
+        expect(content).not.toContain("oh-my-opencode")
+      })
+    })
+  })
+
+  describe("#given opencode.json contains bare oh-my-opencode entry", () => {
+    describe("#when migrating the config", () => {
+      it("#then replaces with oh-my-openagent", () => {
+        const configPath = join(testDir, "opencode.json")
+        writeFileSync(configPath, JSON.stringify({ plugin: ["oh-my-opencode"] }, null, 2))
+
+        const result = migrateLegacyPluginEntry(configPath)
+
+        expect(result).toBe(true)
+        const content = readFileSync(configPath, "utf-8")
+        expect(content).toContain('"oh-my-openagent"')
+        expect(content).not.toContain("oh-my-opencode")
+      })
+    })
+  })
+
+  describe("#given opencode.json contains pinned oh-my-opencode version", () => {
+    describe("#when migrating the config", () => {
+      it("#then preserves the version pin", () => {
+        const configPath = join(testDir, "opencode.json")
+        writeFileSync(configPath, JSON.stringify({ plugin: ["oh-my-opencode@3.11.0"] }, null, 2))
+
+        const result = migrateLegacyPluginEntry(configPath)
+
+        expect(result).toBe(true)
+        const content = readFileSync(configPath, "utf-8")
+        expect(content).toContain("oh-my-openagent@3.11.0")
+      })
+    })
+  })
+
+  describe("#given opencode.json already uses oh-my-openagent", () => {
+    describe("#when checking for migration", () => {
+      it("#then returns false and does not modify the file", () => {
+        const configPath = join(testDir, "opencode.json")
+        const original = JSON.stringify({ plugin: ["oh-my-openagent@latest"] }, null, 2)
+        writeFileSync(configPath, original)
+
+        const result = migrateLegacyPluginEntry(configPath)
+
+        expect(result).toBe(false)
+        expect(readFileSync(configPath, "utf-8")).toBe(original)
+      })
+    })
+  })
+
+  describe("#given config file does not exist", () => {
+    describe("#when attempting migration", () => {
+      it("#then returns false", () => {
+        const result = migrateLegacyPluginEntry(join(testDir, "nonexistent.json"))
+
+        expect(result).toBe(false)
+      })
+    })
+  })
+})