Add a bundled Kimi Code docs system skill

Problem:
Kimi Code has built-in skill commands, but there is no bundled product-docs skill comparable to Codex's system product-docs help. When users ask about Kimi Code setup, config, MCP, skills, plugins, slash commands, sessions, IDE/ACP, or troubleshooting, agents do not have an explicit Kimi Code self-help route.

Proposal:
Add a System skill scope for product-aware bundled skills. Include a kimi-code-docs system skill with guidance to consult official docs, local docs, and source, and expose it as /kimi-code-docs. Proposed priority: Project > User > Extra > System > Built-in.

Implementation direction:
Use Kimi's existing in-memory bundled skill registration style rather than unpacking files to KIMI_CODE_HOME. The system skill is registered as system://kimi-code-docs at session startup after project/user/extra roots load and before built-in skills register.

Validation from prototype:

• agent-core/protocol/kimi-code typecheck pass
• targeted skill/TUI vitest pass
• docs build pass
• protocol and agent-core build pass

I have a draft implementation ready and can open a PR if this direction is acceptable.

Start by tracing Kimi's existing in-memory bundled skill registration and session-startup ordering, including the agent-core/protocol/kimi-code area mentioned in the validation notes. Confirm the system://kimi-code-docs skill and /kimi-code-docs command are exposed with the proposed priority, then run the targeted skill/TUI vitest checks, typecheck, docs build, and protocol and agent-core builds.