feat(scripts): optional single-segment branch prefix for gitflow (#2202)

* feat(scripts): optional single-segment branch prefix for gitflow

- Add spec_kit_effective_branch_name / Get-SpecKitEffectiveBranchName:
  when branch matches prefix/rest with exactly one slash, validate and
  resolve specs/ using only the rest (e.g. feat/001-my-feature).
- Wire into check_feature_branch, find_feature_dir_by_prefix (bash) and
  Test-FeatureBranch, Find-FeatureDirByPrefix + Get-FeaturePathsEnv (PS).
- Align git extension git-common with core validation; remove unused
  get_feature_dir / Get-FeatureDir helpers.
- Extend tests in test_timestamp_branches.py and test_git_extension.py.

Made-with: Cursor

* Update scripts/powershell/common.ps1

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* fix(ps): align feature-dir resolution errors with bash (no throw under Stop)

Find-FeatureDirByPrefix: on ambiguous prefix matches, write errors to stderr
and return $null instead of throwing, matching find_feature_dir_by_prefix.

Get-FeaturePathsEnv: narrow try/catch to ConvertFrom-Json only; add
Get-FeatureDirFromBranchPrefixOrExit to mirror bash get_feature_paths
(stderr + exit 1) when prefix lookup fails, avoiding unhandled terminating
errors under $ErrorActionPreference = 'Stop' in check-prerequisites,
setup-plan, and update-agent-context.

Made-with: Cursor

* Update tests/test_timestamp_branches.py

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Update extensions/git/scripts/powershell/git-common.ps1

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Update scripts/powershell/common.ps1

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

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Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>