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/bunfig.toml

@@ -0,0 +1,7 @@
+preload = ["@opentui/solid/preload"]
+
+[test]
+preload = ["./test/preload.ts"]
+# timeout is not actually parsed from bunfig.toml (see src/bunfig.zig in oven-sh/bun)
+# using --timeout in package.json scripts instead
+# https://github.com/oven-sh/bun/issues/7789