feat(hermes): Hermes Agent harness support, rebased to a Hermes-only diff

Rebase of PR #1922 onto current dev: the ~14 files of v6.1.0-era
codex/release drift are dropped, the porting-guide edits (stale against
the post-prune rewrite, no Hermes content) are dropped, and the Hermes
surface is kept intact: .hermes-plugin/ (on_session_start bootstrap
injection), tests/hermes/ (20 tests, passing), docs/README.hermes.md,
references/hermes-tools.md, the Platform Adaptation row, README section,
and Python ignores.

Known open items from review, unchanged by this rebase: the injection
mechanism uses ctx.inject_message from on_session_start, which the
official plugin guide does not document (pre_llm_call returning
{"context": ...} is the sanctioned path), skills are not registered via
ctx.register_skill, and the acceptance transcript predates the fix.

Co-authored-by: kumarabd <kumarabd@users.noreply.github.com>

tests/hermes/conftest.py

@@ -0,0 +1,19 @@
+import pytest
+from unittest.mock import MagicMock
+
+
+@pytest.fixture
+def mock_ctx():
+    ctx = MagicMock()
+    ctx._hooks = {}
+    ctx._injected = []
+
+    def register_hook(event, fn):
+        ctx._hooks[event] = fn
+
+    def inject_message(content, role="user"):
+        ctx._injected.append({"content": content, "role": role})
+
+    ctx.register_hook.side_effect = register_hook
+    ctx.inject_message.side_effect = inject_message
+    return ctx