feat: add reverse cross-references from marketing-ideas and customer-research to marketing-plan

The marketing-plan skill references both heavily — Section 12 of every
plan cross-references all 139 marketing-ideas, and Section 2 (Strategic
frame) builds on customer-research output. Adding the reverse pointers
so users browsing those skills can discover the comprehensive-plan
deliverable.

Not adding to product-marketing because it has no Related Skills
section to extend.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

skills/customer-research/SKILL.md

@@ -269,3 +269,4 @@ Don't ask all five at once — lead with #1 and #2, then follow up as needed.
 | Writing cold email using research on pain/trigger | `cold-email` |
 | Translating customer research into an ICP for outbound | `prospecting` |
 | Planning content based on discovered topics | `content-strategy` |
+| Rolling research into a comprehensive marketing plan | `marketing-plan` |

skills/marketing-ideas/SKILL.md

@@ -160,6 +160,7 @@ When recommending ideas, provide for each:
 
 ## Related Skills
 
+- **marketing-plan**: When the user wants a comprehensive plan instead of standalone ideas. Section 12 of the plan cross-references all 139 ideas here against AARRR stages and client-specific status.
 - **programmatic-seo**: For scaling SEO content (#4)
 - **competitors**: For comparison pages (#11)
 - **emails**: For email marketing tactics