docs: correct setup docs and drop private references from comments

[cplieger]

Summary

Setup docs corrected against the code and the shared guides at cplieger/docs (wording, secrets handling, compose comments kept identical between compose.yaml and the README), and comments and tests no longer name private host names or internal documents. No behavior changes.

Tested

• Docs scanner (strict) and markdownlint: clean; README compose block matches compose.yaml; Docker Hub overview within 250 words
• Build and tests (Go), shell suites, shellcheck and shfmt where touched
• A forbidden-term scan over the repo prints nothing
• Two reviewers on different models