[FEATURE]: Support external i18n locale packs (community-driven translations) for skills, tools, and agents

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Describe the enhancement you want to request

Problem

opencode surfaces (Web UI, TUI) display hardcoded English text for skills, tools, and agents (names + descriptions). Non-English users currently cannot localize these without editing source files:

• Skill name/description come read-only from each SKILL.md frontmatter and are served verbatim by GET /skill (AppSkillsResponses → {name, description, location, content}).
• Tool and agent names/descriptions are similarly hardcoded.

Editing source files is not durable: skills/agents installed via packages such as @opengsd/gsd-core are overwritten on every update.

Why plugin hooks can't solve this

• tool.definition — only affects tool definitions sent to the LLM, not the UI.
• experimental.chat.system.transform — only affects the LLM system prompt.
• There is no plugin hook intercepting the /skill (or tool/agent) API responses consumed by the Web UI.

Requested behavior

Follow opencode's existing external-directory convention (like themes/, agents/, commands/) to load community locale packs:

1. Config selects the locale:

{
  "i18n": {
    "locale": "zh-CN"
  }
}

2. External locale pack directory — same pattern as other opencode config dirs:

~/.config/opencode/locales/zh-CN.json
# or
~/.config/opencode/locales/zh-CN/skills.json
                         /zh-CN/tools.json
                         /zh-CN/agents.json

3. Locale pack content — keyed by identifier, overriding name/description for display only:

{
  "skills": {
    "gsd-update":     { "name": "GSD 更新", "description": "更新 GSD 到最新版本并显示变更日志" },
    "code-optimizer": { "description": "深度代码优化审计" }
  },
  "tools": {
    "bash": { "description": "执行 Bash 命令" }
  },
  "agents": {
    "explore": { "description": "快速探索代码库" }
  }
}

4. Community-distributed: locale packs are dropped into the directory by users (or installed from npm). opencode provides the loading + application mechanism; the zh-CN pack itself is community-maintained.

Design constraints

• Overrides apply only to display — must not modify on-disk SKILL.md / agent / tool source files.
• Must survive skill/agent package updates (no re-edit after gsd-update).
• Follow the same precedence/merge convention as other opencode config files.
• Works in Web UI (opencode web) and ideally TUI.

Expected outcome

Non-English users install/place a community locale pack, switch i18n.locale, and see localized skill/tool/agent names and descriptions everywhere — without editing source files and without losing translations on updates.