Add a third-party marks rule: no AWS icons, plain-text nominative use only

[Neaox]

Summary

• Adds a Third-party marks section to the brand guidelines, in the three places they live: brand-guidelines.html (new #third-party-marks section between usage and files), the Storybook Voice page, and AGENTS.md hard rule 9.
• The rule: Overcast names AWS and its services only to state compatibility, in plain text. Service icons are lucide or original glyphs on Overcast's own categorical ramp. No AWS service icons or logos on any surface, not even behind an opt-in, and no lookalikes.
• Approved nominative phrasing is listed alongside: "AWS-compatible", "local AWS emulator", "compatible with Amazon S3", and the disclaimer "not affiliated with or endorsed by Amazon Web Services".

Why

• AWS Trademark Guidelines §13 limits nominative fair use to "plain text only (no logos)"; §10 prohibits imitating AWS trade dress including its "product icons".
• The AWS Architecture Icons package ships with no licence file and is offered for diagrams, not product UIs.
• Simple Icons removed every Amazon/AWS icon in v15.0.0 on the same reading.

Verification

• npm run build && node design-system/react/embed.mjs then git diff --stat on the generated directories: empty, no drift (the CI gate).
• npm run build-storybook succeeds; the built Voice page contains the new section.
• brand-guidelines.html opened locally: section renders with the existing ✓/✗ rule styling; links use the accent colour.

Notes

• A companion PR in overcast-sh/overcast adds a pointer comment at the top of web/src/lib/service-registry.ts and a bullet in web/AGENTS.md, both linking to brand.overcast.sh/brand-guidelines.html#third-party-marks.