fix: use global branch numbering instead of per-short-name detection (#1757)

* fix: remove per-short-name number detection from specify prompt

The specify.md prompt instructed the AI to search for existing branches
filtered by the exact short-name, causing every new feature to start at
001 since no branches matched the new short-name. The underlying
create-new-feature.sh script already has correct global numbering logic
via check_existing_branches() that searches ALL branches and spec
directories.

The fix removes the AI's flawed number-detection steps and tells it to
NOT pass --number, letting the script auto-detect the next globally
available number.

Closes #1744
Closes #1468

🤖 Generated with [Claude Code](https://claude.com/code)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix: clarify --json flag requirement per Copilot review

- Rephrased step 2 to mention both --short-name and --json flags
- Added explicit note to always include the JSON flag for reliable output parsing

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>