feat(observability): observer-grade telemetry hooks + NeMo-Relay plugin

Adds backend-neutral observer hooks for plugins: session, turn, API
request, tool, approval, and subagent lifecycle events with stable
correlation IDs (session_id, task_id, turn_id, api_request_id,
tool_call_id, parent/child subagent ids). Extends VALID_HOOKS with
api_request_error and subagent_start.

Hot path is zero-cost when no plugin subscribes: has_hook()/presence
checks gate all payload construction, request payloads are returned
by reference when no middleware rewrites, and the sanitized response
payload no longer embeds raw response objects.

Bundles the optional NeMo-Relay observability plugin
(plugins/observability/nemo_relay) as an in-repo consumer of the new
hooks, peer to the existing langfuse plugin. Fails open when the
optional nemo-relay package is not installed.

Authored-by: Bryan Bednarski <bbednarski@nvidia.com>
Salvaged from #29722 onto current main.

website/docs/user-guide/features/hooks.md

@@ -353,6 +353,9 @@ Gateway hooks only fire in the **gateway** (Telegram, Discord, Slack, WhatsApp,
 
 [Plugins](/user-guide/features/plugins) can register hooks that fire in **both CLI and gateway** sessions. These are registered programmatically via `ctx.register_hook()` in your plugin's `register()` function.
 
+For plugin packaging and registration details, see
+the [Plugins guide](/docs/user-guide/features/plugins).
+
 ```python
 def register(ctx):
     ctx.register_hook("pre_tool_call", my_tool_observer)
@@ -368,6 +371,7 @@ def register(ctx):
 - Callbacks receive **keyword arguments**. Always accept `**kwargs` for forward compatibility — new parameters may be added in future versions without breaking your plugin.
 - If a callback **crashes**, it's logged and skipped. Other hooks and the agent continue normally. A misbehaving plugin can never break the agent.
 - Two hooks' return values affect behavior: [`pre_tool_call`](#pre_tool_call) can **block** the tool, and [`pre_llm_call`](#pre_llm_call) can **inject context** into the LLM call. All other hooks are fire-and-forget observers.
+- Observer callbacks receive `telemetry_schema_version` automatically. When present, `turn_id`, `api_request_id`, `task_id`, `session_id`, and `api_call_count` are separate correlation fields. Treat `api_request_id` as an opaque identifier; do not parse its string format.
 
 ### Quick reference