test(hermes): realign suite with the pre_llm_call mechanism; slim docs to the README section

The 20-test suite still exercised the dead on_session_start/inject_message
mechanism (17 failures against the rewritten plugin). Rewritten for the
real contract: pre_llm_call registration + first-turn-only context return,
register_skill receiving pathlib.Path (the conftest mock now raises on str,
mirroring hermes' AttributeError that silently disables a plugin), both
install layouts resolving skills, loud failure when skills are missing,
tool mapping sourced verbatim from hermes-tools.md, and a bootstrap-size
guard against hermes' 10k-char context spill threshold. 19 tests, passing.

Install docs collapse into the README section per maintainer direction:
docs/README.hermes.md and .hermes-plugin/INSTALL.md are gone; the README
carries the two-line install plus the compaction caveat. plugin.yaml
version aligned to 6.1.1.

tests/hermes/conftest.py

@@ -1,3 +1,5 @@
+from pathlib import Path
+
 import pytest
 from unittest.mock import MagicMock
 
@@ -6,14 +8,23 @@ from unittest.mock import MagicMock
 def mock_ctx():
     ctx = MagicMock()
     ctx._hooks = {}
-    ctx._injected = []
+    ctx._skills = {}
 
     def register_hook(event, fn):
         ctx._hooks[event] = fn
 
-    def inject_message(content, role="user"):
-        ctx._injected.append({"content": content, "role": role})
+    def register_skill(name, path):
+        # Mimic hermes' real register_skill, which calls path.exists() and
+        # therefore breaks on a str (the bug that silently disabled the whole
+        # plugin, found 2026-07-23). Keeping that fidelity here means a
+        # regression to str paths fails these tests instead of failing
+        # silently inside hermes.
+        if not isinstance(path, Path):
+            raise AttributeError(
+                f"register_skill requires a pathlib.Path, got {type(path).__name__}"
+            )
+        ctx._skills[name] = path
 
     ctx.register_hook.side_effect = register_hook
-    ctx.inject_message.side_effect = inject_message
+    ctx.register_skill.side_effect = register_skill
     return ctx