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/agent/prompt/summary.txt

@@ -0,0 +1,11 @@
+Summarize what was done in this conversation. Write like a pull request description.
+
+Rules:
+- 2-3 sentences max
+- Describe the changes made, not the process
+- Do not mention running tests, builds, or other validation steps
+- Do not explain what the user asked for
+- Write in first person (I added..., I fixed...)
+- Never ask questions or add new questions
+- If the conversation ends with an unanswered question to the user, preserve that exact question
+- If the conversation ends with an imperative statement or request to the user (e.g. "Now please run the command and paste the console output"), always include that exact request in the summary