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/util/data-url.test.ts

@@ -0,0 +1,14 @@
+import { describe, expect, test } from "bun:test"
+import { decodeDataUrl } from "../../src/util/data-url"
+
+describe("decodeDataUrl", () => {
+  test("decodes base64 data URLs", () => {
+    const body = '{\n  "ok": true\n}\n'
+    const url = `data:text/plain;base64,${Buffer.from(body).toString("base64")}`
+    expect(decodeDataUrl(url)).toBe(body)
+  })
+
+  test("decodes plain data URLs", () => {
+    expect(decodeDataUrl("data:text/plain,hello%20world")).toBe("hello world")
+  })
+})