[Security] Harden the MVP before enabling Funnel

[troyscott]

Outcome

Complete an evidence-backed security review before the public URL is enabled.

Acceptance criteria

• Public process has read-only filesystem permissions.
• Host, CSRF, cookie, CSP, and Markdown-sanitization controls are tested.
• Source fetching passes allowlist and redirect tests.
• Secrets and private reader data are absent from public responses and Git history.
• Funnel is enabled only after the release checklist passes.