Release 1.12.3 agent usability cleanup

[abhim-dv]

Summary

• Clarify the agent MCP and CLI workflows, document the complete artifact tool surface, and describe internal HTTP transport separately from agent calls.
• Improve first-run MCP setup, startup errors, CLI help and wake/wait terminal outcomes.
• Ship version 1.12.3 with the native Go monitor bundled in platform wheels.

Checks

• Full local Python suite running; cross-platform PR CI is the release gate.
• go test ./... passed.
• uv build --wheel and isolated Windows wheel smoke passed (vanth --version, vanth-monitor --version).
• compileall and git diff --check passed.