Split marketing notes into a research-altitude hub

[AdamSpitz]

Summary

Move marketing notes into specs/product/marketing/, add plan/posture/friend-walkthroughs, and keep marketing.md as a stub.

Cherry-picked from a diverged misc branch onto current dev. CauseStarter links were updated to Commonality so docs-link check passes.

Test plan

• Docs-only; npm run check:docs-links ran in pre-commit.

[AdamSpitz]

Review receipt.

Marketing notes hub split; docs-only. CauseStarter links retargeted to Commonality so docs-link check passes. No blocking findings.

Reviewed-with: grok
Reviewed-files: 13
Reviewed-commit: 358a0be

[AdamSpitz]

Review receipt.

Re-review after rebase onto origin/dev (PR #190 landed). Marketing hub docs; no blocking findings.

Reviewed-with: grok
Reviewed-files: 13
Reviewed-commit: 829c6f2

[AdamSpitz]

Review receipt.

Re-review after rebase onto origin/dev (PRs #190 and #191 landed). Marketing hub docs; no blocking findings.

Reviewed-with: grok
Reviewed-files: 13
Reviewed-commit: 7aa486e