Fix hook priority conflicts and standardize on_Binary naming

on_Snapshot priority fixes:
- redirects.bg.js stays at 31, staticfile.bg.js → 32
- headers.js stays at 55, readability.py → 56
- mercury.py → 57, htmltotext.py → 58

on_Binary hooks now have numeric priorities:
- 10: npm_install.py
- 11: pip_install.py
- 12: brew_install.py
- 13: apt_install.py
- 14: custom_install.py
- 15: env_install.py

archivebox/plugins/env/on_Binary__15_env_install.py

@@ -0,0 +1,71 @@
+#!/usr/bin/env python3
+"""
+Check if a binary is already available in the system PATH.
+
+This is the simplest "provider" - it doesn't install anything,
+it just discovers binaries that are already installed.
+
+Usage: on_Binary__install_using_env_provider.py --binary-id=<uuid> --machine-id=<uuid> --name=<name>
+Output: Binary JSONL record to stdout if binary found in PATH
+
+Environment variables:
+    MACHINE_ID: Machine UUID (set by orchestrator)
+"""
+
+import json
+import os
+import sys
+
+import rich_click as click
+from abx_pkg import Binary, EnvProvider
+
+
+@click.command()
+@click.option('--machine-id', required=True, help="Machine UUID")
+@click.option('--binary-id', required=True, help="Dependency UUID")
+@click.option('--name', required=True, help="Binary name to find")
+@click.option('--binproviders', default='*', help="Allowed providers (comma-separated)")
+def main(binary_id: str, machine_id: str, name: str, binproviders: str):
+    """Check if binary is available in PATH and record it."""
+
+    # Check if env provider is allowed
+    if binproviders != '*' and 'env' not in binproviders.split(','):
+        click.echo(f"env provider not allowed for {name}", err=True)
+        sys.exit(0)  # Not an error, just skip
+
+    # Use abx-pkg EnvProvider to find binary
+    provider = EnvProvider()
+    try:
+        binary = Binary(name=name, binproviders=[provider]).load()
+    except Exception as e:
+        click.echo(f"{name} not found in PATH: {e}", err=True)
+        sys.exit(1)
+
+    if not binary.abspath:
+        click.echo(f"{name} not found in PATH", err=True)
+        sys.exit(1)
+
+    machine_id = os.environ.get('MACHINE_ID', '')
+
+    # Output Binary JSONL record to stdout
+    record = {
+        'type': 'Binary',
+        'name': name,
+        'abspath': str(binary.abspath),
+        'version': str(binary.version) if binary.version else '',
+        'sha256': binary.sha256 or '',
+        'binprovider': 'env',
+        'machine_id': machine_id,
+        'binary_id': binary_id,
+    }
+    print(json.dumps(record))
+
+    # Log human-readable info to stderr
+    click.echo(f"Found {name} at {binary.abspath}", err=True)
+    click.echo(f"  version: {binary.version}", err=True)
+
+    sys.exit(0)
+
+
+if __name__ == '__main__':
+    main()