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/session/prompt/build-switch.txt

@@ -0,0 +1,5 @@
+<system-reminder>
+Your operational mode has changed from plan to build.
+You are no longer in read-only mode.
+You are permitted to make file changes, run shell commands, and utilize your arsenal of tools as needed.
+</system-reminder>