CEO should reuse existing GitHub issues instead of creating duplicates

[lukeinglis]

Problem

When the CEO pipeline executes step 2c (Create GitHub Issue), it always creates a new issue, even when the hypothesis explicitly references an existing one (e.g., "issue #352"). This produces duplicate issues with near-identical content.

Example: Issue #352 already had a full spec for factory handoff. The CEO created #369 with the same content during the experiment pipeline, then #369 had to be closed as a duplicate.

Expected Behavior

Before creating a new issue, the CEO should:

1. Check if the hypothesis text or backlog item references an existing issue number (e.g., #352, issue #352, Addresses: #352)
2. If an existing issue is found, verify it's open: gh issue view <number> --json state
3. If open, use that issue number as $ISSUE_NUM and skip creation
4. Only create a new issue if no existing reference is found

Where to Fix

This is a prompt change in factory/agents/prompts/ceo.md, step 2c (Create GitHub Issue). Add a pre-check before the gh issue create block.

Acceptance Criteria

• CEO checks for existing issue references before creating new issues
• Existing open issues are reused, not duplicated
• New issues are still created when no reference exists