fix(gastown): prevent duplicate bead prompt

[evanjacobson]

Summary

Gastown's Mayor prompt did not explicitly tell the coordinator to avoid duplicate bead creation, which could leave reviewers and users sorting through repeated work items. This adds a concise duplicate-prevention instruction to the existing slinging guidance and protects it with a focused prompt test following the nearby Vitest prompt-test pattern.

Details

• Adds the duplicate-bead instruction near the Mayor's primary bead creation guidance.
• Adds a prompt regression test beside the existing prompt tests.

Verification

N/A, prompt-only change.

Visual Changes

N/A

Reviewer Notes

The worktree still has an unrelated local kilo.json modification that is not included in this PR.

[kilo-code-bot]

Code Review Summary

Status: No Issues Found | Recommendation: Merge

Files Reviewed (2 files)

• services/gastown/src/prompts/mayor-system.prompt.ts
• services/gastown/src/prompts/mayor-system.prompt.test.ts

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_{Reviewed by gpt-5.4-20260305 · Input: 51.2K · Output: 5.5K · Cached: 300K}

_{Review guidance: REVIEW.md from base branch main}