fix(hookify): make package import independent of the install directory name

[ozdemirsarman]

Fixes #69665
Fixes #81448

The problem

The hook entry points make the hookify package importable by putting
os.path.dirname(CLAUDE_PLUGIN_ROOT) on sys.path and relying on the plugin
directory being named exactly hookify.

A marketplace install does not satisfy that — the plugin is unpacked into a
versioned directory (.../hookify@0.1.0), so hookify is not a name Python can
resolve there. Every hook event hits the ImportError branch and the plugin emits
Hookify import error: No module named 'hookify' instead of evaluating any rule.
The plugin looks installed but is inert, on every single hook event.

Repro

With a real rule at .claude/hookify.dangerous-rm.local.md and input
{"tool_name":"Bash","tool_input":{"command":"rm -rf /"}}:

# plugin at .../hookify@0.1.0  (marketplace layout)
before: {"systemMessage": "Hookify import error: No module named 'hookify'"}
after:  {"systemMessage": "**[block-dangerous-rm]** ..."}

The fix

Rather than inferring the package name from the directory layout, register the
package explicitly with its __path__ pinned to the plugin root
(hooks/_bootstrap.py). That makes the import independent of the directory name.

It also removes the dependency on CLAUDE_PLUGIN_ROOT being set and correct: the
root falls back to a path derived from __file__ when the variable is missing or
does not point at a hookify checkout.

Testing

• All four entry points (PreToolUse, PostToolUse, Stop, UserPromptSubmit) under a
  hookify@0.1.0 directory.
• CLAUDE_PLUGIN_ROOT unset — works.
• CLAUDE_PLUGIN_ROOT pointing at a nonexistent path — works.
• Plain hookify/ directory layout — unchanged, still works.