fix: Remove unsupported style separators from Mermaid subgraph declarations (#974)

GitHub's Mermaid renderer doesn't support the `:::` style separator on
subgraph declarations, causing parse errors. Removed `:::futureBlock`
from ProcessMining subgraph and `:::safetyBlock` from SafetyGate subgraph.

The individual nodes within these subgraphs still retain their styles
(e.g., `:::future`), so visual styling is preserved where supported.

Co-authored-by: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: abrichr

README.md

@@ -151,7 +151,7 @@ flowchart TB
             LOADER --> TRAIN --> CKPT
         end
 
-        subgraph ProcessMining["Process Mining"]:::futureBlock
+        subgraph ProcessMining["Process Mining"]
             ABSTRACT["Abstract"]:::future
             PATTERNS["Patterns"]:::future
             ABSTRACT --> PATTERNS
@@ -175,7 +175,7 @@ flowchart TB
             GROUND --> ACT
         end
 
-        subgraph SafetyGate["Safety Gate"]:::safetyBlock
+        subgraph SafetyGate["Safety Gate"]
             VALIDATE["Validate"]
             CONFIRM["Confirm"]:::future
             VALIDATE --> CONFIRM