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>

website/docs/reference/environment-variables.md

@@ -156,7 +156,7 @@ For native Anthropic auth, Hermes prefers Claude Code's own credential files whe
 
 ### Langfuse Observability
 
-Environment variables for the bundled [`observability/langfuse`](/docs/user-guide/features/built-in-plugins#observabilitylangfuse) plugin. Set these with `hermes tools → Langfuse Observability` or manually in `~/.hermes/.env`. The plugin must also be enabled (`hermes plugins enable observability/langfuse`) before any of these take effect.
+Environment variables for the bundled [`observability/langfuse`](/docs/user-guide/features/built-in-plugins#observabilitylangfuse) plugin. Set these in `~/.hermes/.env`. The plugin must also be enabled (`hermes plugins enable observability/langfuse`, or check the box in `hermes plugins`) before any of these take effect.
 
 | Variable | Description |
 |----------|-------------|

website/docs/user-guide/features/built-in-plugins.md

@@ -121,22 +121,14 @@ Traces Hermes turns, LLM calls, and tool invocations to [Langfuse](https://langf
 
 The plugin is fail-open: no SDK installed, no credentials, or a transient Langfuse error — all turn into a silent no-op in the hook. The agent loop is never impacted.
 
-**Setup (interactive — recommended):**
-
-```bash
-hermes tools          # → Langfuse Observability → Cloud or Self-Hosted
-```
-
-The wizard collects your keys, `pip install`s the `langfuse` SDK, and adds `observability/langfuse` to `plugins.enabled` for you. Restart Hermes and the next turn ships a trace.
-
-**Setup (manual):**
+**Setup:**
 
 ```bash
 pip install langfuse
 hermes plugins enable observability/langfuse
 ```
 
-Then put the credentials in `~/.hermes/.env`:
+Or check the box in the interactive `hermes plugins` UI. Then put the credentials in `~/.hermes/.env`:
 
 ```bash
 HERMES_LANGFUSE_PUBLIC_KEY=pk-lf-...