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.

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[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adds behavior main does not have (for example send-time /skill-id expansion details), otherwise it can be closed as superseded.

[vastsa]

I verified this against current main at f7e82876. The requested user-skill slash menu and send-time expansion are already present in the current composer flow, including the skill command kind and the Skills autocomplete group, and are covered by the current desktop tests and specs.

This branch is also conflicting with main, and its implementation targets the pre-refactor electron/main/index.ts. Merging it would reintroduce superseded structure and duplicate behavior. I am leaving this PR unmerged because the requested behavior is already present in main.