docs: add local c-node install entry

[Kot141078]

Adds a README section for installing a local c-node candidate.

Scope:

• adds a clear How to install c entry
• keeps c-node boundaries explicit: not chatbot, not model endpoint, not cloud assistant
• links to the public install guide path on ivankotov.eu/install-c/
• keeps Codex / coding agents framed as bounded installer/reviewer assistants, not memory owners or authority holders

Boundary:

• documentation-only
• no runtime changes
• no secrets or private memory
• no conformance or deployment claim

Changed file: README.md only.