fix(plugins): remove 8KB heuristic limit on memory-provider detection

The auto-coercion heuristic that detects memory provider plugins by
scanning for `register_memory_provider` or `MemoryProvider` in the
plugin's __init__.py was limited to the first 8192 bytes. Some
third-party memory plugins (e.g. mempalace) place the registration
call later in the file, causing the heuristic to miss them and load
them as general plugins — which crashes because PluginContext does
not expose `register_memory_provider`.

The fix is to read the entire file instead of a fixed-size prefix.
Plugin __init__.py files are small; the performance impact is
negligible. The same fix is applied to both the general plugin
auto-coercion (hermes_cli/plugins.py) and the memory-specific
discovery helper (plugins/memory/__init__.py).

Fixes #990 — mempalace plugin now auto-detected as exclusive and
routed to the memory provider discovery path correctly.

plugins/memory/__init__.py

@@ -58,7 +58,7 @@ def _is_memory_provider_dir(path: Path) -> bool:
     if not init_file.exists():
         return False
     try:
-        source = init_file.read_text(errors="replace")[:8192]
+        source = init_file.read_text(errors="replace")
         return "register_memory_provider" in source or "MemoryProvider" in source
     except Exception:
         return False