feat: list user skills in the composer "/" menu and expand them on send

[yexisu]

Fixes #142

Problem

Typing / in the composer only showed app commands and plugin commands. Active user skills (~/.agents/skills and <project>/.agents/skills) never appeared, on any OS — Windows users reported it first (#142).

Root cause: IPC.invoke.composerCommands merges only builtin aliases, prompt templates, plugin commands, and extension commands. The user-skill registry (host-core skills.active) was never consulted for the menu.

Fix

• Add a skill kind to ComposerCommand and append active user skills to the / namespace (after extension commands), scope-filtered by the open workspace like every other app-facing surface.
• New "Skill commands" group in the autocomplete panel with its own i18n label (all 8 locales) and the sparkles icon.
• On send, /skill-id extra text expands to the same <skill> block the Skill tool returns, with the extra text appended as additional instructions; unknown /names stay literal text, and templates keep priority.

Test

• Workspace build (incl. TS type-check) passes; packages/agent-runtime: 348 tests pass.
• Desktop node --test failures on this machine are pre-existing on unmodified main (macOS packaging, fs-scope symlink cases) — verified identical before/after.

Specs updated: 04-ux/04-builtin-commands.md §7 (EN + zh-CN mirror) and E2E-249 in the E2E test plan.

[vercel]

@yexisu is attempting to deploy a commit to the vastsa's projects Team on Vercel.

A member of the Team first needs to authorize it.

[vastsa]

Current main already lists active user/plugin skills in the / menu (8e838549 feat(composer): expose active skills in slash menu) and #142 is closed on that evidence. This PR duplicates that landing; please rebase only if it still adds behavior main does not have (for example send-time /skill-id expansion details), otherwise it can be closed as superseded.