fix: block Podman on macOS and remove ghcr.io default (#39)

* fix: block Podman on macOS and remove ghcr.io default

- Add platform check in PodmanCLI.__init__() that raises FuzzForgeError
  on macOS with instructions to use Docker instead
- Change RegistrySettings.url default from "ghcr.io/fuzzinglabs" to ""
  (empty string) for local-only mode since no images are published yet
- Update _ensure_module_image() to show helpful error when image not
  found locally and no registry configured
- Update tests to mock Linux platform for Podman tests
- Add root ruff.toml to fix broken configuration in fuzzforge-runner

* rewrite guides for module architecture and update repo links

---------

Co-authored-by: AFredefon <antoinefredefon@yahoo.fr>

ruff.toml

@@ -0,0 +1,20 @@
+line-length = 120
+
+[lint]
+select = [ "ALL" ]
+ignore = [
+    "COM812",   # conflicts with the formatter
+    "D100",     # ignoring missing docstrings in public modules
+    "D104",     # ignoring missing docstrings in public packages
+    "D203",     # conflicts with 'D211'
+    "D213",     # conflicts with 'D212'
+    "TD002",    # ignoring missing author in 'TODO' statements
+    "TD003",    # ignoring missing issue link in 'TODO' statements
+]
+
+[lint.per-file-ignores]
+"tests/*" = [
+    "ANN401",   # allowing 'typing.Any' to be used to type function parameters in tests
+    "PLR2004",  # allowing comparisons using unamed numerical constants in tests
+    "S101",     # allowing 'assert' statements in tests
+]