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/src/global/index.ts

@@ -0,0 +1,54 @@
+import fs from "fs/promises"
+import { xdgData, xdgCache, xdgConfig, xdgState } from "xdg-basedir"
+import path from "path"
+import os from "os"
+import { Filesystem } from "../util/filesystem"
+
+const app = "opencode"
+
+const data = path.join(xdgData!, app)
+const cache = path.join(xdgCache!, app)
+const config = path.join(xdgConfig!, app)
+const state = path.join(xdgState!, app)
+
+export namespace Global {
+  export const Path = {
+    // Allow override via OPENCODE_TEST_HOME for test isolation
+    get home() {
+      return process.env.OPENCODE_TEST_HOME || os.homedir()
+    },
+    data,
+    bin: path.join(cache, "bin"),
+    log: path.join(data, "log"),
+    cache,
+    config,
+    state,
+  }
+}
+
+await Promise.all([
+  fs.mkdir(Global.Path.data, { recursive: true }),
+  fs.mkdir(Global.Path.config, { recursive: true }),
+  fs.mkdir(Global.Path.state, { recursive: true }),
+  fs.mkdir(Global.Path.log, { recursive: true }),
+  fs.mkdir(Global.Path.bin, { recursive: true }),
+])
+
+const CACHE_VERSION = "21"
+
+const version = await Filesystem.readText(path.join(Global.Path.cache, "version")).catch(() => "0")
+
+if (version !== CACHE_VERSION) {
+  try {
+    const contents = await fs.readdir(Global.Path.cache)
+    await Promise.all(
+      contents.map((item) =>
+        fs.rm(path.join(Global.Path.cache, item), {
+          recursive: true,
+          force: true,
+        }),
+      ),
+    )
+  } catch (e) {}
+  await Filesystem.write(path.join(Global.Path.cache, "version"), CACHE_VERSION)
+}