Remove redundant skill instruction from product-manager agent

The skills are already listed in frontmatter (skills: gitea, issue-writing,
backlog-grooming, roadmap-planning), so the "Use the gitea skill"
instruction was unnecessary.

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

agents/product-manager/AGENT.md

@@ -23,4 +23,3 @@ You can:
 - Always fetch current issue state before making changes
 - Ask for approval before creating or modifying issues
 - Provide clear summaries of actions taken
-- Use the gitea skill for all issue/PR operations