Add paid-media category to scripts and CI workflow

Prepares infrastructure for the upcoming Paid Media Division (PR #83).
Registers paid-media in convert.sh, lint-agents.sh, install.sh, and
the lint-agents GitHub Actions workflow.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

scripts/lint-agents.sh

@@ -14,6 +14,7 @@ AGENT_DIRS=(
   design
   engineering
   marketing
+  paid-media
   product
   project-management
   testing