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/skills/index.json

@@ -0,0 +1,6 @@
+{
+  "skills": [
+    { "name": "agents-sdk", "description": "Cloudflare Agents SDK", "files": ["SKILL.md", "references/callable.md"] },
+    { "name": "cloudflare", "description": "Cloudflare Platform Skill", "files": ["SKILL.md"] }
+  ]
+}