fix: use global branch numbering instead of per-short-name detection

[fsilvaortiz]

Summary

• The specify.md prompt instructed the AI to search for existing branches filtered by exact short-name, so every new feature started at 001 since no branches matched the new short-name
• The underlying create-new-feature.sh already has correct global numbering logic via check_existing_branches() that searches ALL branches and spec directories
• Simplified step 2 to tell the AI to NOT pass --number, letting the script auto-detect the next globally available number

Closes #1744
Closes #1468

Root cause

The prompt at step 2b told the AI to grep for branches matching the exact short-name:

git ls-remote --heads origin | grep -E 'refs/heads/[0-9]+-<short-name>$'

And step 2c said: "If no existing branches/directories found with this short-name, start with number 1". Since each new feature has a unique short-name, the search always returned empty and numbering reset to 001.

Test plan

• Verified the script correctly assigns 001, 002, 003 to 3 different features when --number is NOT passed
• Reproduced the bug: passing --number 1 manually causes duplicate 001 prefixes (confirming the AI override was the problem)
• Full test suite passes (120 tests)
• Verified the change is a net removal of 24 lines of flawed logic, replaced with 4 lines of clear instructions

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[Copilot]

Pull request overview

Updates the /speckit.specify command prompt to rely on the existing create-new-feature scripts’ global branch/spec numbering logic, avoiding accidental resets to 001 caused by short-name–scoped detection.

Changes:

• Removes the prompt’s manual branch-number detection steps (remote/local/spec-dir scanning).
• Instructs running the feature creation script without --number, letting it auto-select the next global number.
• Simplifies the “IMPORTANT” guidance to prevent AI-driven overrides of numbering.

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[mnriem]

@fsilvaortiz Can you address the Copilot suggestions where applicable. And what about the PowerShell variant of the script?

[fsilvaortiz]

@mnriem Applied the Copilot suggestion in b6bccdd — clarified that --json must also be included, not just --short-name.

Regarding PowerShell: the bug is not in the scripts — both create-new-feature.sh and create-new-feature.ps1 already have correct global numbering logic. The PowerShell script's Get-NextBranchNumber (line 108) explicitly searches "ALL branches (not just matching short name)" when -Number is not passed. The bug was solely in the prompt telling the AI to manually detect a per-short-name number and override --number/-Number. The updated prompt already includes the PowerShell example without -Number (line 45).

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