[Bug] agent-message and agent-observe skill docs missing import section and typed API signatures

[Xsidz]

Problem

SKILL.md files for agent-message and agent-observe lack import instructions and typed function signatures. Agents using these skills have to guess at the API, and some non-existent functions (e.g. get_agent_ids()) are easy to hallucinate.

Fix

Add to each SKILL.md:

• ## Import section with the correct import path
• ## API Reference with typed Python signatures for each exported function
• A list of functions that do NOT exist (to prevent hallucination)
• Fix invalid Python typing syntax in code examples (str | None not "str" | None)

PR #920 implements this fix. Posting here per the new discussion-first process.