docs: convert OODA loop diagram to real Mermaid syntax

The operating-model diagram in README was ASCII box art in a plain code
fence, not an actual Mermaid diagram — it wouldn't render as a graph on
Gitea/GitHub. Replaced with a `flowchart TD` matching the convention already
used by oikos/gen-topology.py's generated topology.md.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -46,60 +46,25 @@ Before you act, **classify your action against [oikos/policy.yaml](oikos/policy.
 
 ### The Oikos OODA Loop + Decision Tree
 
-```
-┌─────────────────────────────────────────────────────────────┐
-│                    Observe                                  │
-│        (probes, drift detectors, agent signals)             │
-└────────────────────┬────────────────────────────────────────┘
-                     │
-┌────────────────────▼────────────────────────────────────────┐
-│                    Orient                                   │
-│      (ontology, context, state, entity relations)           │
-└────────────────────┬────────────────────────────────────────┘
-                     │
-┌────────────────────▼────────────────────────────────────────┐
-│  Decide: Classify against oikos/policy.yaml                │
-│                                                              │
-│  ├─ read_only                → [unattended, no ledger]      │
-│  ├─ reversible_low           → [unattended + ledger entry]  │
-│  ├─ config_mutation          → [ESCALATE: operator approval]│
-│  └─ destructive              → [ESCALATE + confirmation]    │
-└────────────────────┬────────────────────────────────────────┘
-                     │
-        ┌────────────┴────────────┐
-        │                         │
-   ┌────▼─────┐            ┌──────▼──────┐
-   │ Auto-act │            │  Escalate   │
-   └────┬─────┘            │ (get approval
-        │                  │  via homelab │
-┌───────▼──────────────────┴──────────┐  │
-│            Act                       │  │
-│  (homelab CLI, runbooks, skills)    │  │
-└────────────────┬─────────────────────┘  │
-                 │                        │
-            ┌────▼────────────────────────┘
-            │
-┌───────────▼──────────────────────────────┐
-│          Verify                           │
-│    (checklist from SKILL.md)              │
-└───────────┬──────────────────────────────┘
-            │
-┌───────────▼──────────────────────────────┐
-│          Ledger                           │
-│     (mutation record: who/what/risk)      │
-└───────────┬──────────────────────────────┘
-            │
-┌───────────▼──────────────────────────────┐
-│        Document                           │
-│   (wiki update, same-session rule)        │
-└───────────┬──────────────────────────────┘
-            │
-            └─────────────────┐
-                              │
-                    ┌─────────▼─────────┐
-                    │ Loop back to      │
-                    │    Observe        │
-                    └───────────────────┘
+```mermaid
+flowchart TD
+    Observe["**Observe**<br/>probes, drift detectors, agent signals"]
+    Orient["**Orient**<br/>ontology, context, state, entity relations"]
+    Decide{"**Decide**<br/>classify against oikos/policy.yaml"}
+    Auto["Auto-act<br/>(unattended)"]
+    Escalate["Escalate<br/>homelab approval request"]
+    Act["**Act**<br/>homelab CLI, runbooks, skills"]
+    Verify["**Verify**<br/>checklist from SKILL.md"]
+    Ledger["**Ledger**<br/>mutation record: who/what/risk"]
+    Document["**Document**<br/>wiki update, same-session rule"]
+
+    Observe --> Orient --> Decide
+    Decide -->|read_only, reversible_low| Auto
+    Decide -->|config_mutation, destructive| Escalate
+    Auto --> Act
+    Escalate -->|approval granted| Act
+    Act --> Verify --> Ledger --> Document
+    Document -.loop.-> Observe
 ```
 
 ### Risk Classes (enforced, not advisory)