feat(plugins): add optional-plugins/ discovery + langfuse_tracing as first official optional plugin

Introduces optional-plugins/ — a new category for plugins that ship with
the repo but are NOT auto-discovered. They live alongside the code but only
land in ~/.hermes/plugins/ (and thus get loaded) when the user explicitly
installs them.

Core changes:
- optional-plugins/observability/langfuse-tracing/ — langfuse tracing plugin
  (pre/post LLM + tool hooks, usage/cost normalization, fail-open when SDK
  missing). NOT in plugins/ so zero import overhead on devices that don't
  want it.
- hermes_cli/plugins_cmd.py — official install path: _resolve_official_plugin()
  recognises 'official/<category>/<name>' identifiers and copies from
  optional-plugins/ into ~/.hermes/plugins/ (no git clone, no network).
  _list_official_plugins() enumerates available optional plugins.
  cmd_list(available=True) shows not-yet-installed official plugins.
- hermes_cli/main.py — hermes plugins list --available flag
- hermes_cli/tools_config.py — Langfuse Observability in TOOL_CATEGORIES;
  post_setup handler installs the langfuse SDK and runs cmd_install()
- hermes_cli/config.py — Langfuse credentials in OPTIONAL_ENV_VARS;
  optional tuning keys in _EXTRA_ENV_KEYS

User flows:
  hermes plugins install official/observability/langfuse-tracing
  hermes plugins list --available
  hermes tools  (-> Langfuse Observability -> credentials -> auto-installs)

Closes #15764

optional-plugins/observability/langfuse/plugin.yaml

@@ -0,0 +1,14 @@
+name: langfuse
+version: "1.0.0"
+description: "Optional Langfuse observability for Hermes — traces conversations, LLM calls, and tool usage. Install via: hermes plugins install official/observability/langfuse"
+author: NousResearch
+requires_env:
+  - HERMES_LANGFUSE_PUBLIC_KEY
+  - HERMES_LANGFUSE_SECRET_KEY
+hooks:
+  - pre_api_request
+  - post_api_request
+  - pre_llm_call
+  - post_llm_call
+  - pre_tool_call
+  - post_tool_call