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.

pyproject.toml

@@ -124,6 +124,7 @@ honcho = ["honcho-ai==2.0.1"]
 # extra that exposes a Starlette-backed server surface so pip/uv can't resolve
 # a vulnerable pre-1.0.1 transitive. Bump in lockstep with uv.lock.
 mcp = ["mcp==1.26.0", "starlette==1.0.1"]  # starlette: CVE-2026-48710
+nemo-relay = ["nemo-relay==0.3"]
 homeassistant = ["aiohttp==3.13.3"]
 sms = ["aiohttp==3.13.3"]
 # Computer use — macOS background desktop control via cua-driver (MCP stdio).
@@ -244,6 +245,7 @@ plugins = [
   # <platform>" (#34034), web-search providers go missing (#28149), etc.
   "**/plugin.yaml",
   "**/plugin.yml",
+  "**/README.md",
 ]
 
 [tool.setuptools.packages.find]