Summary

When --number is explicitly provided to create-new-feature.sh, this PR adds validation to ensure the specified prefix number is not already in use by another spec directory.

Problem

Even after PR #1237 fixed the auto-detection logic, users can still create collisions by explicitly using the --number option:

# Existing spec
specs/001-feature-a/

# User runs with explicit --number
./create-new-feature.sh --number 1 --short-name "feature-b" "Description"

# Result: COLLISION!
specs/001-feature-b/  # Same prefix as 001-feature-a!

Solution

Added validation after FEATURE_NUM is calculated:

• Checks all existing spec directories for the same prefix
• Allows retry of the same feature (same suffix)
• Blocks different features from using duplicate prefixes
• Provides helpful error message guiding users to auto-detection

Test

$ ./create-new-feature.sh --number 1 --short-name "conflict" "Test"
Error: Prefix 001 is already used by '001-existing-feature'
Hint: Remove --number option to auto-detect the next available number

Related

Related to #1332 - This addresses a remaining edge case not covered by #1237

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