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/changelog.md

@@ -0,0 +1,5 @@
+go through each PR merged since the last tag
+
+for each PR spawn a subagent to summarize what the PR was about. focus on user facing changes. if it was entirely internal or code related you can ignore it. also skip docs updates. each subagent should append its summary to UPCOMING_CHANGELOG.md
+
+once that is done, read UPCOMING_CHANGELOG.md and group it into sections for better readability. make sure all PR references are preserved