feat(agent-core): add Kimi Code docs system skill

[starSumi]

Summary

• add a System skill scope and register bundled system skills after project/user/extra roots and before built-ins
• add an in-memory kimi-code-docs system skill exposed as /kimi-code-docs
• document System skill priority and the new slash command in English and Chinese docs

Closes #1172

Validation

• pnpm --filter @moonshot-ai/protocol run typecheck
• pnpm --filter @moonshot-ai/agent-core run typecheck
• pnpm --filter @moonshot-ai/kimi-code run typecheck
• pnpm --filter @moonshot-ai/agent-core exec vitest run test/skill/scanner.test.ts test/skill/registry.test.ts test/harness/skill-session.test.ts
• pnpm --filter @moonshot-ai/kimi-code exec vitest run test/tui/commands/skills.test.ts
• pnpm --filter @moonshot-ai/protocol run build
• pnpm --filter @moonshot-ai/agent-core run build
• pnpm -C docs run build
• git diff --check origin/main

[changeset-bot]

🦋 Changeset detected

Latest commit: e9f117f

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@moonshot-ai/kimi-code	Minor

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