Add configurable git branch templates for monorepos

[PascalThuet]

Summary

• Adds git extension branch_template config with {author}, {app}, {number}, and {slug} tokens, plus branch_prefix as a shorthand namespace.
• Applies the template in both Bash and PowerShell feature-branch scripts while keeping the default {number}-{slug} behavior unchanged.
• 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.

Implementation note

This follows the concrete use case and change-surface analysis in #3081 (comment). That comment is relevant because it identifies the desired {author}/{app}/{number}-{slug} shape and notes that downstream commands already resolve feature directories through .specify/feature.json / SPECIFY_FEATURE_DIRECTORY, so branch creation and validation are the main places that need to become namespace-aware.

Validation

• .venv/bin/python -m pytest tests/extensions/git/test_git_extension.py tests/test_timestamp_branches.py -q

Disclosure

Prepared on behalf of @PascalThuet by Codex (model: GPT-5, autonomous). The 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