feat: add phase 3 prompt customization backend

- parse prompt model and argument-hint metadata and persist model on prompt invocations
- expand markdown-linked file context in scanned prompt and instruction bodies
- discover .claude/rules instructions and support Claude-style paths metadata
- discover customization roots up to the nearest parent repository and watch them for changes
- apply per-turn prompt model overrides before sidecar execution

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

sidecar/src/Aryx.AgentHost/Services/AgentInstructionComposer.cs

@@ -100,6 +100,11 @@ internal static class AgentInstructionComposer
             lines.Add($"Agent: {promptInvocation.Agent.Trim()}");
         }
 
+        if (!string.IsNullOrWhiteSpace(promptInvocation.Model))
+        {
+            lines.Add($"Model: {promptInvocation.Model.Trim()}");
+        }
+
         if (promptInvocation.Tools is not null)
         {
             List<string> toolNames = promptInvocation.Tools