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/fixture/fixture.test.ts

@@ -0,0 +1,26 @@
+import { $ } from "bun"
+import { describe, expect, test } from "bun:test"
+import fs from "fs/promises"
+import { tmpdir } from "./fixture"
+
+describe("tmpdir", () => {
+  test("disables fsmonitor for git fixtures", async () => {
+    await using tmp = await tmpdir({ git: true })
+
+    const value = (await $`git config core.fsmonitor`.cwd(tmp.path).quiet().text()).trim()
+    expect(value).toBe("false")
+  })
+
+  test("removes directories on dispose", async () => {
+    const tmp = await tmpdir({ git: true })
+    const dir = tmp.path
+
+    await tmp[Symbol.asyncDispose]()
+
+    const exists = await fs
+      .stat(dir)
+      .then(() => true)
+      .catch(() => false)
+    expect(exists).toBe(false)
+  })
+})