refactor: apply minimal tfcode branding

- Rename packages/opencode → packages/tfcode (directory only)
- Rename bin/opencode → bin/tfcode (CLI binary)
- Rename .opencode → .tfcode (config directory)
- Update package.json name and bin field
- Update config directory path references (.tfcode)
- Keep internal code references as 'opencode' for easy upstream sync
- Keep @opencode-ai/* workspace package names

This minimal branding approach allows clean merges from upstream
opencode repository while providing tfcode branding for users.

packages/tfcode/test/plugin/auth-override.test.ts

@@ -0,0 +1,56 @@
+import { describe, expect, test } from "bun:test"
+import path from "path"
+import fs from "fs/promises"
+import { tmpdir } from "../fixture/fixture"
+import { Instance } from "../../src/project/instance"
+import { ProviderAuth } from "../../src/provider/auth"
+import { ProviderID } from "../../src/provider/schema"
+
+describe("plugin.auth-override", () => {
+  test("user plugin overrides built-in github-copilot auth", async () => {
+    await using tmp = await tmpdir({
+      init: async (dir) => {
+        const pluginDir = path.join(dir, ".opencode", "plugin")
+        await fs.mkdir(pluginDir, { recursive: true })
+
+        await Bun.write(
+          path.join(pluginDir, "custom-copilot-auth.ts"),
+          [
+            "export default async () => ({",
+            "  auth: {",
+            '    provider: "github-copilot",',
+            "    methods: [",
+            '      { type: "api", label: "Test Override Auth" },',
+            "    ],",
+            "    loader: async () => ({ access: 'test-token' }),",
+            "  },",
+            "})",
+            "",
+          ].join("\n"),
+        )
+      },
+    })
+
+    await using plain = await tmpdir()
+
+    const methods = await Instance.provide({
+      directory: tmp.path,
+      fn: async () => {
+        return ProviderAuth.methods()
+      },
+    })
+
+    const plainMethods = await Instance.provide({
+      directory: plain.path,
+      fn: async () => {
+        return ProviderAuth.methods()
+      },
+    })
+
+    const copilot = methods[ProviderID.make("github-copilot")]
+    expect(copilot).toBeDefined()
+    expect(copilot.length).toBe(1)
+    expect(copilot[0].label).toBe("Test Override Auth")
+    expect(plainMethods[ProviderID.make("github-copilot")][0].label).not.toBe("Test Override Auth")
+  }, 30000) // Increased timeout for plugin installation
+})