Support namespaced git feature branch templates

[PascalThuet]

Summary

• Adds git extension branch_template config with {author}, {app}, {number}, and {slug} tokens, plus branch_prefix as a shorthand namespace.
• Applies templates in both Bash and PowerShell feature-branch scripts while preserving the default {number}-{slug} branch shape.
• Makes feature number extraction, branch-number scanning, and validation tolerant of path-style branch namespaces.

Why

Monorepo installs can have separate Spec Kit projects but one shared root Git branch namespace. This lets projects opt into names like jdoe/app-a/008-guided-tour without requiring separate Git repositories or changing downstream feature path resolution.

Fixes #3081.

Issue context

The implementation follows the concrete monorepo use case described in #3081 (comment). That comment is relevant because it identifies the target branch shape ({author}/{app}/{number}-{slug}) and points out that downstream commands already resolve feature directories through .specify/feature.json / SPECIFY_FEATURE_DIRECTORY, so branch creation and validation are the important places to make namespace-aware.

TDD sequence

• dbe9203 adds failing coverage for branch templates, branch-prefix shorthand, namespace-scoped numbering, namespaced GIT_BRANCH_NAME extraction, and nested-prefix validation.
• f48178d implements the behavior and documentation that make those tests pass.

Validation

• Red check after the test commit: 5 targeted tests failed as expected.
• Green check after implementation: .venv/bin/python -m pytest tests/extensions/git/test_git_extension.py tests/test_timestamp_branches.py -q
• Result: 105 passed, 42 skipped.

Disclosure

Prepared on behalf of @PascalThuet by Codex (model: GPT-5, autonomous). Each commit includes an Assisted-by: trailer.

[mnriem]

I realize this still a draft, but I wanted to make sure that you should merge the template into the git-config.yml for ease of use

[PascalThuet]

Thanks, done in the latest push. branch_template is now included in both git-config.yml and config-template.yml.

It defaults to "", which preserves the existing {number}-{slug} behavior. Users can set it to {author}/{app}/{number}-{slug} for monorepo-style branch names.