feat(plugins): add bundled observability/langfuse plugin

Opt-in Langfuse tracing for Hermes conversations — LLM calls, tool
usage, usage/cost breakdown per span. Hooks into pre/post_api_request,
pre/post_llm_call, pre/post_tool_call. SDK is optional; missing SDK or
credentials renders the plugin inert.

Salvaged from PR #16845 by @kshitijk4poor, who wrote the plugin
(~875 LOC, 6 hooks, Langfuse usage-details/cost-details normalization,
read_file payload summarization).

Salvage scope (why this isn't PR #16845 as-authored):
- Lives at plugins/observability/langfuse/ (standalone kind, opt-in via
  plugins.enabled) instead of a new parallel optional-plugins/
  directory. Standalone bundled plugins are already opt-in — only their
  plugin.yaml is scanned at startup; the Python module is not imported
  unless the user enables it. The premise of optional-plugins/ (avoid
  import cost for users who don't want it) is already solved by the
  existing plugin system.
- Dropped the triple activation gate (plugins.enabled +
  plugins.langfuse.enabled + HERMES_LANGFUSE_ENABLED). The Hermes plugin
  system's own enable/disable is authoritative; runtime credentials
  gate whether the hook actually traces.
- Rewrote _is_enabled() → cached _get_langfuse() with an _INIT_FAILED
  sentinel. The original called hermes_cli.config.load_config() from
  every hook invocation (full yaml parse + deep merge + env expansion
  on every pre/post_tool_call, potentially 100+ times per turn). The
  cached version reads env once and returns the cached client or None
  on every subsequent call with zero further work.
- hermes tools → Langfuse Observability post-setup adds
  observability/langfuse to plugins.enabled directly (via
  _save_enabled_set) instead of going through an install-copy flow.

Enable:
  hermes tools                                        # interactive
  hermes plugins enable observability/langfuse        # manual

Required env (set by `hermes tools` or in ~/.hermes/.env):
  HERMES_LANGFUSE_PUBLIC_KEY
  HERMES_LANGFUSE_SECRET_KEY
  HERMES_LANGFUSE_BASE_URL                            # optional

Co-authored-by: kshitijk4poor <kshitijk4poor@gmail.com>

hermes_cli/tools_config.py

@@ -425,6 +425,31 @@ TOOL_CATEGORIES = {
             },
         ],
     },
+    "langfuse": {
+        "name": "Langfuse Observability",
+        "icon": "📊",
+        "providers": [
+            {
+                "name": "Langfuse Cloud",
+                "tag": "Hosted Langfuse (cloud.langfuse.com)",
+                "env_vars": [
+                    {"key": "HERMES_LANGFUSE_PUBLIC_KEY", "prompt": "Langfuse public key (pk-lf-...)", "url": "https://cloud.langfuse.com"},
+                    {"key": "HERMES_LANGFUSE_SECRET_KEY", "prompt": "Langfuse secret key (sk-lf-...)", "url": "https://cloud.langfuse.com"},
+                ],
+                "post_setup": "langfuse",
+            },
+            {
+                "name": "Langfuse Self-Hosted",
+                "tag": "Self-hosted Langfuse instance",
+                "env_vars": [
+                    {"key": "HERMES_LANGFUSE_PUBLIC_KEY", "prompt": "Langfuse public key (pk-lf-...)"},
+                    {"key": "HERMES_LANGFUSE_SECRET_KEY", "prompt": "Langfuse secret key (sk-lf-...)"},
+                    {"key": "HERMES_LANGFUSE_BASE_URL", "prompt": "Langfuse server URL (e.g. http://localhost:3000)", "default": "http://localhost:3000"},
+                ],
+                "post_setup": "langfuse",
+            },
+        ],
+    },
 }
 
 # Simple env-var requirements for toolsets NOT in TOOL_CATEGORIES.
@@ -567,6 +592,40 @@ def _run_post_setup(post_setup_key: str):
                 _print_info("      git submodule update --init --recursive")
                 _print_info('      uv pip install -e "./tinker-atropos"')
 
+    elif post_setup_key == "langfuse":
+        # Install the langfuse SDK.
+        try:
+            __import__("langfuse")
+            _print_success("    langfuse SDK already installed")
+        except ImportError:
+            import subprocess
+            _print_info("    Installing langfuse SDK...")
+            result = subprocess.run(
+                [sys.executable, "-m", "pip", "install", "langfuse", "--quiet"],
+                capture_output=True, text=True, timeout=120,
+            )
+            if result.returncode == 0:
+                _print_success("    langfuse SDK installed")
+            else:
+                _print_warning("    langfuse SDK install failed — run manually: pip install langfuse")
+        # Opt the bundled observability/langfuse plugin into plugins.enabled.
+        # The plugin ships in the repo but doesn't load until the user enables
+        # it (standalone plugins are opt-in).
+        try:
+            from hermes_cli.plugins_cmd import _get_enabled_set, _save_enabled_set
+            enabled = _get_enabled_set()
+            if "observability/langfuse" in enabled or "langfuse" in enabled:
+                _print_success("    Plugin observability/langfuse already enabled")
+            else:
+                enabled.add("observability/langfuse")
+                _save_enabled_set(enabled)
+                _print_success("    Plugin observability/langfuse enabled")
+        except Exception as exc:
+            _print_warning(f"    Could not enable plugin automatically: {exc}")
+            _print_info("    Run manually: hermes plugins enable observability/langfuse")
+        _print_info("    Restart Hermes for tracing to take effect.")
+        _print_info("    Verify: hermes plugins list")
+
 
 # ─── Platform / Toolset Helpers ───────────────────────────────────────────────