Add onboarding message action demo

[FanyinLiu]

Summary

• Add a separate final onboarding step for a static received-message action demo.
• Keep the demo conceptual: broad source notice, local desktop hint, and user-chosen actions only.
• Add guard tests so the demo stays schema-free, deterministic, locale-safe, and disconnected from notification/chat/autonomy runtime paths.

Pro review

• Web ChatGPT Pro reviewed the original onboarding and notification privacy source first.
• Pro recommended a separate message_action_demo step plus a static helper instead of adding this to CompanionStep.
• Pro flagged wording that felt too much like a real notification pipeline; I weakened the flow to received -> hint -> decide and removed schema/ID/timestamp language from the user-facing copy.
• Pro confirmed P0 runtime/privacy/schema risks were closed and said the change is safe for Draft PR.

Visual check

• Opened the local Vite preview and re-opened onboarding from Settings.
• Walked to step 6 in a narrow panel viewport and adjusted the layout so the privacy note and all three action labels remain visible above the footer.

Verification

• node --experimental-strip-types --test tests/onboarding-guide-support.test.ts
• npm run i18n:audit
• npx tsc -b --pretty false
• npm run lint
• npm run message-privacy:audit
• npm run companion-boundary:audit
• npm run v04:draft-stack:audit:quick
• git diff --check
• npm run verify:pr

No release, tag, or stable README state was changed.