chore: remove unused opencode tooling config

### Overview
Removes OpenCode agent/command tooling config that is no longer used in this repo, and gitignores locally-downloaded docker-compose dev assets that were untracked but not ignored.

### Intent
The real intent of this issue is:
> Local workspace cleanup surfaced a stale branch with these deletions already made but uncommitted; formalizing the removal instead of leaving it as dangling local state.

### Source of Truth
feat/docker-compose branch had .opencode/ and opencode.jsonc deleted locally, uncommitted, alongside ~30MB of untracked .docker/downloaded/ binaries with no matching .gitignore entry.

### Tasks
- [ ] Remove .opencode/ directory and opencode.jsonc
- [ ] Remove .github/workflows/opencode.yml
- [ ] Add .docker/downloaded/ to .gitignore

### Acceptance Criteria
- Repo no longer contains OpenCode tooling config
- .docker/downloaded/ is gitignored

### Definition of Done
- Change merged to master

Inspect the .opencode/ directory, opencode.jsonc, .github/workflows/opencode.yml, and .gitignore on the feat/docker-compose branch. Remove the unused OpenCode files and add .docker/downloaded/ to .gitignore, then verify those paths are absent or ignored and the repository no longer tracks the tooling configuration.

由索引模型根據 Issue 內容生成。