[codex] update billing guide and simplify docs navigation

[HynLcc]

Summary

• Updates billing docs and screenshots for the current settings flow.
• Standardizes help-center screenshot presentation with baked dotted-background PNGs and shared docs-screenshot styling.
• Replaces selected Field, Base, and SSO screenshots with staged processed assets.
• Simplifies Basic navigation labels/routes, including Base, Field overview, and Authority Matrix cleanup.
• Removes the AI Field practical guide from navigation.
• Adds AGENT.md notes for future screenshot processing and ignores local handoff/scratch image files.
• Fixes existing Automation documentation links so Mintlify broken-link checks pass.

Validation

• git diff --check
• mintlify validate
• mintlify broken-links
• Local MDX image reference check: 446 local /images/... refs, 0 missing
• Staged screenshot check: 67 tracked images/staged-* PNGs, 67 refs, 0 missing/unreferenced