fix(gator): retry review after draft blocker clears

[johntmyers]

Summary

Allow gator to run the independent reviewer once after a same-head-SHA draft-only blocker clears and the PR is ready for review.

Related Issue

N/A

Changes

• Detect prior draft-only ## Blocked gator dispositions in the same-SHA duplicate guard.
• Ignore those draft-only blockers only after the current PR is no longer draft.
• Document the exception in the gator prompt and gator-gate skill.
• Add guard tests for ready vs still-draft behavior.

Testing

• scripts/agents/gator/bin/gh_guard_test.sh passes
• bash -n scripts/agents/gator/bin/gh scripts/agents/gator/bin/gh_guard_test.sh scripts/agents/run.sh passes
• git diff --check passes
• mise run pre-commit passes
• Unit tests added/updated
• E2E tests added/updated (not applicable)

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)