Remove the legacy upstream .opencode/ directory from the repo #177

Context

Raised in review of #176. The repo still ships the upstream opencode project directory .opencode/ (37 files):

• agent/: docs, duplicate-pr, hush, translator, triage
• command/: ai-deps, commit, issues, learn, rmslop, spellcheck
• a glossary/ of UI translations
• AGENTS.md, env.d.ts, .gitignore

Nothing in this repo's CI or docs uses them. translator.md still references models.dev flags (#40).

Problem

The directory is not just dead weight; the CLI reads parts of it:

• With AICTRL_HEADLESS=true, the instruction loader reads only .opencode/AGENTS.md (session/instruction.ts:18-21). Headless runs against this repo therefore get upstream opencode instructions.
• .opencode is one of the external directories scanned for skills (skill/skill.ts EXTERNAL_DIRS).

Proposal

• Delete .opencode/ from the repo.
• Separately, decide whether the CLI should keep treating .opencode/ paths specially at all. The instruction loader's .opencode/AGENTS.md special case belongs with the headless prompt-contract work in AICTRL_HEADLESS define is a no-op; full and lite builds send different system prompts #152/Headless: pi-style minimal prompt — one base prompt and functional tool descriptions (eval-gated) #158.

Acceptance

• git ls-files .opencode is empty.
• No test or script depends on it.