fix: silence git fetch stdout to prevent branch number parse error

[fsilvaortiz]

Summary

Fixes create-new-feature.sh --json failing with:

line 250: 10#Fetching: value too great for base (error token is "10#Fetching")

Root cause: git fetch --all --prune 2>/dev/null only silences stderr. When git fetch writes status text (e.g., Fetching origin...) to stdout, it gets captured by command substitution in check_existing_branches(), contaminating BRANCH_NUMBER with non-numeric text. The subsequent $((10#$BRANCH_NUMBER)) then fails.

Fix: Redirect both stdout and stderr to /dev/null:

- git fetch --all --prune 2>/dev/null || true
+ git fetch --all --prune >/dev/null 2>&1 || true

Closes #1592

Test plan

• Verified the fix silences both stdout and stderr from git fetch
• All tests pass (100/100 — 95 existing + 5 new)
• Added 5 new tests in tests/test_create_new_feature.py:
  • test_json_output_is_valid — JSON output has correct keys
  • test_feature_num_is_numeric — FEATURE_NUM is always zero-padded numeric
  • test_script_does_not_leak_git_fetch_stdout — no fetch noise in stdout
  • test_script_does_not_leak_git_fetch_to_json — repeated runs produce clean JSON
  • test_git_fetch_redirect_pattern_in_script — regression guard for the redirect pattern (no git required)
• Tests that require git are marked with pytest.mark.skipif for environments without git

AI Assistance Disclosure

This PR was drafted with assistance from Claude Code (Anthropic). The root cause analysis, fix, and tests were validated manually by the author.

🤖 Generated with Claude Code