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/Contracts/ProtocolModels.cs

@@ -200,6 +200,7 @@ public sealed class RunTurnPromptInvocationDto
     public string ResolvedPrompt { get; init; } = string.Empty;
     public string? Description { get; init; }
     public string? Agent { get; init; }
+    public string? Model { get; init; }
     public IReadOnlyList<string>? Tools { get; init; }
 }
 

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

sidecar/tests/Aryx.AgentHost.Tests/AgentInstructionComposerTests.cs

@@ -210,6 +210,7 @@ public sealed class AgentInstructionComposerTests
                 SourcePath = @".github\prompts\docs\doc-review.prompt.md",
                 Description = "Review docs for missing steps",
                 Agent = "plan",
+                Model = "Claude Sonnet 4.5",
                 Tools = ["view", "glob"],
                 ResolvedPrompt = "Review the docs for missing steps and propose updates."
             });
@@ -219,6 +220,7 @@ public sealed class AgentInstructionComposerTests
         Assert.Contains("Name: doc-review", instructions, StringComparison.Ordinal);
         Assert.Contains("Description: Review docs for missing steps", instructions, StringComparison.Ordinal);
         Assert.Contains("Agent: plan", instructions, StringComparison.Ordinal);
+        Assert.Contains("Model: Claude Sonnet 4.5", instructions, StringComparison.Ordinal);
         Assert.Contains("Tools: view, glob", instructions, StringComparison.Ordinal);
         Assert.Contains(
             "Prompt instructions:\nReview the docs for missing steps and propose updates.",