feat: add git-setup-remotes and git-sign-push CLIs

Ship the two maintainer workflow helpers as installable Python CLIs so
any fork-based OSS project can reuse them without vendoring shell
scripts:

  * git-setup-remotes configures origin/fork/main-tracking/pushDefault
    for a fork-based clone. URLs are parameterized (--canonical / --fork
    or CANONICAL_URL / FORK_URL) so the same binary bootstraps any
    maintainer's repo. Idempotent.
  * git-sign-push GPG-signs every unpushed commit on the current branch
    and pushes, resolving the target remote from remote.pushDefault
    (falling back to origin) for branches without upstream.

Both refuse to run when CLAUDE_CODE/CLAUDECODE is set, since the Claude
sandbox blocks .git/config writes and access to ~/.gnupg: failing fast
beats failing late.

Other additions:
  * Makefile: install / install-dev / test / lint / clean targets.
  * .github/workflows/test.yml: pytest + ruff matrix for py 3.10/11/12.
  * MIRRORS: github, git.veen.world:2201, code.infinito.nexus:2201, pypi.
  * LICENSE switched to MIT; README records the extraction origin
    (s.infinito.nexus/code) and adds author + license sections.
  * Tests cover sandbox-refusal guards and the fork-URL resolution
    preference order (CLI arg > env var > existing fork remote > origin
    when not canonical).

pyproject.toml

@@ -5,17 +5,25 @@ build-backend = "setuptools.build_meta"
 [project]
 name = "git-maintainer-tools"
 version = "0.1.0"
-description = ""
+description = "Small CLIs (git-setup-remotes, git-sign-push) for fork-based OSS maintainer workflows."
 readme = "README.md"
 requires-python = ">=3.10"
 authors = [{ name = "Kevin Veen-Birkenbach", email = "kevin@veen.world" }]
-license = { text = "All rights reserved by Kevin Veen-Birkenbach" }
+license = { text = "MIT" }
 urls = { Homepage = "https://github.com/kevinveenbirkenbach/git-maintainer-tools" }
 
 dependencies = []
 
+[project.optional-dependencies]
+dev = ["pytest>=7", "ruff>=0.4"]
+
+[project.scripts]
+git-setup-remotes = "git_maintainer_tools.setup_remotes:main"
+git-sign-push = "git_maintainer_tools.sign_push:main"
+
 [tool.setuptools]
 package-dir = {"" = "src"}
 
 [tool.setuptools.packages.find]
 where = ["src"]
+include = ["git_maintainer_tools*"]