feat: collapse expanded skill blocks in user messages

[wuxiangru915]

Summary

pi's agent core expands /skill:name commands by inlining the entire SKILL.md into the user message (<skill name="..." location="...">...full content...</skill>). In the web UI that floods the conversation with verbose skill instructions every time a skill is invoked.

This PR renders each expanded <skill> block in user messages as a collapsible header that defaults to collapsed — showing just skill: <name>, the SKILL.md location, and the line count — with the full content available on click.

Behavior

• Collapsed by default: one compact header line per skill block (▸ skill: name /path/to/SKILL.md 177 行 · 展开).
• Click to expand: reveals the full skill content, rendered as before (markdown body).
• Text around skill blocks is preserved and rendered in order; messages without skill blocks are untouched.
• Copy action still copies the full original message text.

Implementation

• lib/skill-block.ts (new): splitSkillBlocks(text) splits a message into <skill> blocks and plain text segments (name/location parsed from attributes, order-independent). Unit-tested in lib/skill-block.test.mjs.
• components/MessageView.tsx: UserMessageView renders skill segments via a new SkillBlock collapsible component; non-skill segments keep the existing MarkdownBody rendering.
• i18n: adds chat.lines (en/zh) and reuses existing i18n.expand/i18n.collapse.

Verification

• Unit tests: lib/skill-block.test.mjs (7) + components/MessageView.test.mjs (3 new SSR render tests) + i18n registry — all passing.
• tsc --noEmit, eslint, production next build all clean.
• End-to-end in a real browser: a real session containing an expanded skill block renders the collapsed header (no skill body visible), and clicking it reveals the full content.