feat(telemetry)!: upload one record per execution node and one closure per chain

Core no longer assembles a whole trace locally. Every workflow node is uploaded
the moment it completes as its own `execution_node` record, and each execution
closes with exactly one `execution_chain` record; all records of one execution
carry the same `event_id`, so Space groups them back into a single chain.

- every execution starts at an inbound event: event-less Pipeline lanes and
  WebUI/API debug runs synthesize their virtual inbound event at the boundary
  and are marked debug=true, so a Runner is never a trace origin
- sampling is decided once per event from a stable hash of the event id (the
  previous `int(id[:8], 16)` raised on every `platform:`-prefixed id and so
  dropped every successful inbound-event chain) and is reported on each record
  as sample{mode,denominator}; the default mode is now `all`
- the local trace buffer, the 120s TTL, the per-process trace cap and the shared
  open-node mirror are gone, which also removes the truncated-chain upload and
  the re-buffered-closed-trace leak they caused
- a broken chain is always uploaded, and its reason falls back to the first
  failed node so the chain explains itself
- flush keeps records until the sender accepts them and counts what it drops;
  telemetry health replaces the unused send-task queue
- heartbeat reports bound Agent / plugin-processor / event-binding counts
- bound Pipeline and Agent resources (plugins, MCP servers, knowledge bases,
  tools with their Host source, skills, models and the Runner schema selectors
  that picked them) are reported on the chain record
- failure text is bounded and redacted instead of passing exception text through
- the dead per-query feature counters and their call sites are removed

Wire contract: schema 2 (`execution_node` / `execution_chain`).

src/langbot/pkg/api/http/service/agent.py

@@ -348,25 +348,42 @@ class AgentService:
                 execution_events.append(visible_result)
 
         final_text = ''
-        async for output in self.ap.agent_run_orchestrator.run(
-            event,
-            binding,
-            adapter_context={
-                '_query': execution_query,
-                '_execution_context': execution_context,
-                '_result_observer': observe_result,
-            },
+        # A debug run has no platform event: the synthetic event envelope is the
+        # execution's inbound boundary, so the chain is opened here with the real
+        # event id and marked as a debug WebUI/API execution. The Runner inside
+        # then joins this chain instead of becoming a trace origin of its own.
+        from ....telemetry.execution import ingress as execution_ingress
+
+        principal_type = str(getattr(getattr(context, 'principal', None), 'principal_type', '') or '')
+        origin = 'api' if principal_type == 'api_key' else 'webui'
+        with execution_ingress(
+            self.ap,
+            'event_done',
+            execution_context,
+            event_id,
+            debug=True,
+            origin=origin,
+            synthetic_event=event_type,
         ):
-            output_text = self._provider_output_to_text(output)
-            if output_text:
-                final_text = output_text
-            output_items.append(
-                {
-                    'kind': output.__class__.__name__,
-                    'role': str(getattr(output, 'role', '') or ''),
-                    'text': output_text,
-                }
-            )
+            async for output in self.ap.agent_run_orchestrator.run(
+                event,
+                binding,
+                adapter_context={
+                    '_query': execution_query,
+                    '_execution_context': execution_context,
+                    '_result_observer': observe_result,
+                },
+            ):
+                output_text = self._provider_output_to_text(output)
+                if output_text:
+                    final_text = output_text
+                output_items.append(
+                    {
+                        'kind': output.__class__.__name__,
+                        'role': str(getattr(output, 'role', '') or ''),
+                        'text': output_text,
+                    }
+                )
 
         return {
             'event_id': event_id,