fix(plugins): remove unreachable hermes tools → Langfuse path

The langfuse plugin is hooks-only (no toolsets), so it never appears in
`hermes tools` — that menu iterates `_get_effective_configurable_toolsets()`
(= `CONFIGURABLE_TOOLSETS` + plugin-registered toolsets), and "langfuse"
is in neither. The `TOOL_CATEGORIES["langfuse"]` setup wizard (with its
`post_setup: "langfuse"` hook that pip-installs the SDK and writes
`plugins.enabled`) was reachable only when a toolset key "langfuse" got
enabled, which can't happen — so it's been dead code, and the docs that
promised "Setup (interactive): hermes tools → Langfuse Observability"
were silently broken.

Right home for that wizard is `hermes plugins` (e.g. auto-running a
plugin's post-setup hook on enable), which is a generic plugin-setup
mechanism worth designing properly rather than shoehorning langfuse
back into `hermes tools`. Until that exists, point users at the
working manual flow.

Code:
- Delete `TOOL_CATEGORIES["langfuse"]` (24 lines) — unreachable.
- Delete the `post_setup_key == "langfuse"` branch in `_run_post_setup`
  (29 lines) — only caller was the deleted TOOL_CATEGORIES entry.

Docs / comments (point at the manual flow + interactive `hermes plugins`):
- `plugins/observability/langfuse/README.md`: collapse the two-option
  setup section to the single working flow.
- `plugins/observability/langfuse/plugin.yaml`: update `description`.
- `plugins/observability/langfuse/__init__.py`: update module docstring.
- `hermes_cli/config.py`: update inline comment above the LANGFUSE_*
  env-var allow-list.
- `website/docs/user-guide/features/built-in-plugins.md`: collapse
  "Setup (interactive)" + "Setup (manual)" into one accurate block.
- `website/docs/reference/environment-variables.md`: update the
  cross-reference in the Langfuse env-vars section.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

plugins/observability/langfuse/__init__.py

@@ -4,11 +4,11 @@ Traces Hermes conversations, LLM calls, and tool usage to Langfuse.
 
 Activation is handled by the Hermes plugin system — standalone plugins only
 load when listed in ``plugins.enabled`` (via ``hermes plugins enable
-observability/langfuse`` or ``hermes tools → Langfuse Observability``). At
-runtime the plugin also requires the ``langfuse`` SDK and credentials; if
-either is missing the hooks are inert.
+observability/langfuse``, or by checking the box in the interactive
+``hermes plugins`` UI). At runtime the plugin also requires the
+``langfuse`` SDK and credentials; if either is missing the hooks are inert.
 
-Required env vars (set via ``hermes tools`` or ~/.hermes/.env):
+Required env vars (set in ~/.hermes/.env):
   HERMES_LANGFUSE_PUBLIC_KEY  - Langfuse project public key (pk-lf-...)
   HERMES_LANGFUSE_SECRET_KEY  - Langfuse project secret key (sk-lf-...)
   HERMES_LANGFUSE_BASE_URL    - Langfuse server URL (default: https://cloud.langfuse.com)