docs: rename "Security implications" to "Security posture" (#24181)

Renames the "Security implications" section to "Security posture" and
reframes the intro paragraph. "Implications" reads as a caveat or
warning; the section actually describes built-in structural guarantees
of the control plane architecture.

> PR generated with Coder Agents

docs/ai-coder/agents/architecture.md

@@ -233,10 +233,11 @@ Because state lives in the database:
 - The agent can resume work by targeting a new workspace and continuing from the
   last git branch or checkpoint.
 
-## Security implications
+## Security posture
 
-The control plane architecture provides several security advantages for AI
-coding workflows.
+The control plane architecture provides built-in security properties for AI
+coding workflows. These are structural guarantees, not configuration options —
+they hold by default for every agent session.
 
 ### No API keys in workspaces