fix: call python3, not python, so the contrast tooling runs (#11)

Stock macOS and Debian ship python3 only. Nothing named `python` is on
PATH there, so the bundled antislop-contrast MCP server never starts and
the command the antislop-human skill prints fails the same way.

Changes `command` in plugin.json, the allowed-tools Bash rule and the
example in antislop-human/SKILL.md, and the usage strings in
contrast-check.py. The script's own shebang was already python3.

.claude-plugin/plugin.json

@@ -12,7 +12,7 @@
   "skills": ["./skills/"],
   "mcpServers": {
     "antislop-contrast": {
-      "command": "python",
+      "command": "python3",
       "args": ["${CLAUDE_PLUGIN_ROOT}/skills/antislop-human/contrast-mcp.py"],
       "env": {}
     }

skills/antislop-human/SKILL.md

@@ -1,7 +1,7 @@
 ---
 name: antislop-human
 description: "Human and accessibility skill for antislop. Contrast, keyboard, focus, and states for real people. Includes the contrast checker."
-allowed-tools: Bash(python *) Read Write Edit Glob Grep
+allowed-tools: Bash(python3 *) Read Write Edit Glob Grep
 ---
 # antislop-human
 
@@ -50,7 +50,7 @@ The home of the contrast checker. Three layers, from most to least convenient:
 **The script.** When a script runtime is available and the file is present, run it instead of computing by hand. The script ships in this skill's folder (`contrast-check.py`, next to this `SKILL.md`):
 
 ```bash
-python "${CLAUDE_SKILL_DIR}/contrast-check.py" "#FFFFFF" "#777777"
+python3 "${CLAUDE_SKILL_DIR}/contrast-check.py" "#FFFFFF" "#777777"
 # normal text: FAIL (4.48 < 4.5)
 # large text:  PASS (4.48 >= 3.0)
 ```

skills/antislop-human/contrast-check.py

@@ -2,9 +2,9 @@
 """WCAG 2.x contrast checker, home of the antislop-human contrast checker.
 
 Usage:
-    python contrast-check.py "#FFFFFF" "#777777"
-    python contrast-check.py FFFFFF 777777
-    python contrast-check.py --selftest
+    python3 contrast-check.py "#FFFFFF" "#777777"
+    python3 contrast-check.py FFFFFF 777777
+    python3 contrast-check.py --selftest
 
 Prints the contrast ratio and a PASS/FAIL verdict for normal text (4.5:1)
 and large text (3:1, 18px+ per antislop R-25). Exit code 0 only when both
@@ -142,7 +142,7 @@ def main(argv):
     if len(argv) == 1 and argv[0] == "--selftest":
         return selftest()
     if len(argv) != 2:
-        print("usage: python contrast-check.py <hex1> <hex2> | --selftest")
+        print("usage: python3 contrast-check.py <hex1> <hex2> | --selftest")
         return 2
     try:
         ratio = contrast_ratio(parse_hex(argv[0]), parse_hex(argv[1]))