Add public paste CLI and skill

Provide a zero-dependency Python tool that races multiple public paste providers and documents safe AI-tool usage through a project skill.

tests/test_validation.py

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+from __future__ import annotations
+
+import pytest
+
+from publicpaste.validation import ValidationError, body_contains_text_fragment, validate_url
+
+
+@pytest.mark.parametrize(
+    ("provider", "url"),
+    [
+        ("paste.rs", "https://paste.rs/abc123"),
+        ("dpaste.org", "https://dpaste.org/ABC123"),
+        ("0x0.st", "https://0x0.st/abcd.txt"),
+        ("snippet.host", "https://snippet.host/abcdef"),
+        ("paste.centos.org", "https://paste.centos.org/view/12345"),
+        ("termbin.com", "http://termbin.com/abc"),
+    ],
+)
+def test_validate_url_accepts_provider_urls(provider: str, url: str) -> None:
+    assert validate_url(provider, url) == url
+
+
+@pytest.mark.parametrize(
+    ("provider", "url"),
+    [
+        ("paste.rs", "ftp://paste.rs/abc"),
+        ("paste.rs", "https://example.com/abc"),
+        ("0x0.st", "https://0x0.st/"),
+        ("dpaste.org", "https://dpaste.org/a b"),
+    ],
+)
+def test_validate_url_rejects_bad_urls(provider: str, url: str) -> None:
+    with pytest.raises(ValidationError):
+        validate_url(provider, url)
+
+
+def test_body_contains_text_fragment_tolerates_html_escape() -> None:
+    assert body_contains_text_fragment(b"<pre>hello &amp; goodbye</pre>", "hello & goodbye")
+
+
+def test_body_contains_text_fragment_rejects_missing_text() -> None:
+    assert not body_contains_text_fragment("something else", "hello world")