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.

.tfcode/command/spellcheck.md

@@ -0,0 +1,5 @@
+---
+description: spellcheck all markdown file changes
+---
+
+Look at all the unstaged changes to markdown (.md, .mdx) files, pull out the lines that have changed, and check for spelling and grammar errors.