feat(skills): add docs skill and plugin manifest

[h30s]

Resolves #10

Summary

Adds the docs skill and plugin manifest. This skill guides the agent to document code from what it actually does at runtime rather than what it was supposed to do, ground every statement in the code, enforce the skip-the-obvious rule, explicitly name undocumented areas, and report discrepancies between code and documentation as findings rather than resolving them silently.

What task was tested & Before vs. After

• Task: Documenting a service module with in-memory caching and silent timeout fallbacks that contradicted existing JSDoc comments.
• Before: Restated function names tautologically (getUser gets a user), assumed standard error throwing based on comments, and silently resolved code-comment mismatches by guessing intent.
• After: Cited exact file:line references for each guarantee, reported the code-vs-doc drift as a primary finding with risk analysis, highlighted runtime invariants (60s cache, null on 404, fallback behavior), and explicitly stated boundaries on unverified database reconnection paths.

What was deliberately left out

• No speculative architecture documentation: Kept the scope strictly to what is verified in the source code rather than guessing developer intent or unwritten future roadmap.
• No auto-editing or code fixes: Uses tools: chat (read-only by default) to keep documentation review focused on analysis and prevent silently editing implementation code.

Checklist

• Manifest in plugins/docs/.oxcode-plugin/plugin.json declared and valid.
• Registered in .oxcode-plugin/marketplace.json.
• Frontmatter uses capability: Deep analysis and tools: chat (no model names).
• Passes all CI workflow checks.