feat: Introduce Hashline encoding/validation, implement core REPL and chat session logic, and establish initial project structure with a license and editor configuration.

ChatSession.cs

@@ -0,0 +1,59 @@
+using Microsoft.Extensions.AI;
+
+namespace AnchorCli;
+
+internal sealed class ChatSession
+{
+    private readonly IChatClient _agent;
+    public ContextCompactor Compactor { get; }
+    public List<ChatMessage> History { get; }
+
+    public ChatSession(IChatClient innerClient)
+    {
+        Compactor = new ContextCompactor(innerClient);
+        
+        var tools = ToolRegistry.GetTools();
+        _agent = new ChatClientBuilder(innerClient)
+            .UseFunctionInvocation()
+            .Build();
+
+        History = new List<ChatMessage>
+        {
+            new(ChatRole.System, $$"""
+                You are anchor, a coding assistant that edits files using the Hashline technique.
+
+                ## Reading files
+                When you read a file, lines are returned in the format: lineNumber:hash|content
+                The "lineNumber:hash|" prefix is METADATA for anchoring — it is NOT part of the file.
+
+                ## Editing files
+                To edit, reference anchors as "lineNumber:hash" in startAnchor/endAnchor parameters.
+                The newLines/initialLines parameter must contain RAW SOURCE CODE ONLY.
+                ❌ WRONG: ["5:a3|    public void Foo()"]
+                ✅ RIGHT: ["    public void Foo()"]
+                Never include the "lineNumber:hash|" prefix in content you write — it will corrupt the file.
+
+                ## Workflow
+                1. Always read a file before editing it.
+                2. After a mutation, verify the returned fingerprint.
+                3. Edit from bottom to top so line numbers don't shift.
+                4. If an anchor fails validation, re-read the file to get fresh anchors.
+
+                Keep responses concise. You have access to the current working directory.
+                You are running on: {{System.Runtime.InteropServices.RuntimeInformation.OSDescription}}
+            """)
+        };
+    }
+
+    public async IAsyncEnumerable<ChatResponseUpdate> GetStreamingResponseAsync(
+        [System.Runtime.CompilerServices.EnumeratorCancellation] CancellationToken cancellationToken = default)
+    {
+        var options = new ChatOptions { Tools = ToolRegistry.GetTools() };
+        var stream = _agent.GetStreamingResponseAsync(History, options, cancellationToken);
+        
+        await foreach (var update in stream.WithCancellation(cancellationToken))
+        {
+            yield return update;
+        }
+    }
+}