docs: add Mermaid diagrams to improve visual documentation

Enhance documentation with six Mermaid diagrams to better explain
complex system implementations:

- Adaptive polling lifecycle flowchart showing enqueue→execute→feedback
  cycle with scheduler, priority queue, and worker interactions
- Circuit breaker state machine diagram illustrating Closed↔Open↔Half-open
  transitions with triggers and recovery paths
- Temperature proxy architecture diagram highlighting trust boundaries,
  security controls, and data flow between host/container/cluster
- Sensor proxy request flow sequence diagram showing auth, rate limiting,
  validation, and SSH execution pipeline
- Alert webhook pipeline flowchart detailing template resolution, URL
  rendering, HTTP dispatch, and retry logic
- Script library workflow diagram illustrating dev→test→bundle→distribute
  lifecycle emphasizing modular design

These visualizations make it easier for operators and contributors to
understand Pulse's sophisticated architectural patterns.

docs/operations/pulse-sensor-proxy-runbook.md

@@ -9,6 +9,41 @@
 - Limiters: ~12 requests/minute per UID (burst 2), per-UID concurrency 2, global concurrency 8, 2 s penalty on validation failures
 
 ## Monitoring Alerts & Response
+
+```mermaid
+sequenceDiagram
+    participant Backend as Pulse Backend
+    participant Proxy as Sensor Proxy RPC Server
+    participant Limiter as Limiter (per UID & global)
+    participant Validator as Payload Validator
+    participant SSH as Cluster Node (forced `sensors -j`)
+    participant Metrics as Metrics & Audit Log
+
+    Backend->>Proxy: RPC request (get_temperature)
+    Proxy->>Proxy: Extract SO_PEERCRED (UID/GID/PID)
+    Proxy->>Limiter: Check per-UID rate & concurrency
+    alt Rate limit exceeded
+        Limiter-->>Proxy: reject
+        Proxy-->>Backend: 429 Too Many Requests (2 s penalty)
+        Proxy->>Metrics: increment limiter_rejections_total
+    else Allowed
+        Limiter-->>Proxy: permit
+        Proxy->>Validator: Validate method & payload
+        alt Validation failure
+            Validator-->>Proxy: error
+            Proxy-->>Backend: 400 validation error
+            Proxy->>Metrics: penalty + audit log entry
+        else Valid request
+            Validator-->>Proxy: ok
+            Proxy->>SSH: run `sensors -j` via forced command
+            SSH-->>Proxy: temperature JSON
+            Proxy-->>Backend: telemetry payload
+            Proxy->>Metrics: record success, latency histogram
+            Proxy->>Metrics: append audit/audit trail
+        end
+    end
+```
+
 ### Rate Limit Hits (`pulse_proxy_limiter_rejections_total`)
 1. Check audit log entries tagged `limiter.rejection` for offending UID.
 2. Confirm workload legitimacy; if expected, consider increasing limits via config override.