feat(skills): add marketing skills collection

shared/skills/marketing-council/references/advisor-template.md

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+# Custom Advisor Template
+
+Copy this structure to add an advisor to the bench. Save custom advisors to `.agents/advisors/<kebab-name>.md` in your project (not inside the skill folder) so they survive skill updates.
+
+Two kinds of custom advisors, two grounding standards:
+
+- **Public figures** (a famous marketer not on the bench): every framework and position must trace to something they published or said — research before writing, cite sources, follow the same grounding rules as the built-in dossiers.
+- **Private advisors** (your former boss, your best customer, your CFO): the *user* supplies the positions and heuristics. The agent must not invent views for a real private person — interview the user to fill the template.
+
+---
+
+```markdown
+# [Full Name]
+
+**Lens:** [One sentence — the distinct way they see marketing problems.]
+
+## Core frameworks
+
+- **[Framework name]** ([source, year]): [1-2 sentence accurate definition.]
+- …3-6 total. If it's borrowed from someone else, say so.
+
+## Documented positions
+
+- [A strong opinion they actually hold] — *[source]*
+- …5-8 total. Include at least one contrarian position; a persona with
+  no unpopular opinions produces no useful disagreement.
+
+## Signature questions
+
+- [A question they characteristically ask about any marketing problem]
+- …3-5 total. These open the advisor's take in a session.
+
+## Best for / blind spots
+
+**Best for:** [problem types their lens genuinely illuminates]
+**Blind spots:** [documented criticisms or acknowledged limits — this is
+what makes their dissent honest rather than decorative]
+
+## Voice notes
+
+[2-3 sentences: sentence rhythm, favorite metaphors, tone, tics. Enough
+to write in their register without fabricating quotes.]
+
+## Key works
+
+- *[Title]* ([year]) — [one line on what it contributes to the persona]
+```
+
+---
+
+**Seating a custom advisor:** mention them by name when convening ("seat my advisor Maria on this council"). The agent loads the file from `.agents/advisors/` and treats it like any bench dossier, including the grounding rules — no fabricated quotes, no invented endorsements.