feat(mcp): add save_finding to the external server

src/ClaudeDo.Worker/External/FindingsMcpTools.cs

@@ -0,0 +1,54 @@
+using System.ComponentModel;
+using ClaudeDo.Worker.Findings;
+using ClaudeDo.Worker.Git;
+using ModelContextProtocol.Server;
+
+namespace ClaudeDo.Worker.External;
+
+public sealed record SaveFindingResult(
+    bool Saved, string Slug, string List, string Path, int TotalFindings, bool NearCapacity);
+
+[McpServerToolType]
+public sealed class FindingsMcpTools
+{
+    private readonly IFindingsStore _store;
+    private readonly IFindingsStoreLocator _locator;
+
+    public FindingsMcpTools(IFindingsStore store, IFindingsStoreLocator locator)
+    {
+        _store = store;
+        _locator = locator;
+    }
+
+    [McpServerTool, Description(
+        "Record a durable trap or invariant you just learned about this codebase, so future sessions " +
+        "read it instead of rediscovering it — call it the moment you notice one, not at the end of " +
+        "the session. Admission bar (all three must hold, or don't save it): lasting (still true next " +
+        "week), non-obvious (a competent reader would get it wrong), and behaviour-changing (\"X looks " +
+        "like Y but is Z — do W instead\"). A bug you fixed is not a finding, that is git history; " +
+        "neither is a design decision or a status update. Findings are a scarce, curated list, not a " +
+        "log — when unsure, don't save it. Re-using a slug overwrites that finding. This is a dumb " +
+        "write: it does not read the code or generate the finding for you, you already have the " +
+        "context. nearCapacity=true in the result means the index is getting too long to stay cheap — " +
+        "prune stale entries before adding more.")]
+    public async Task<SaveFindingResult> SaveFinding(
+        [Description("Stable lowercase kebab-case id, max 60 chars, e.g. 'conpty-arg-quoting'. Same slug overwrites.")] string slug,
+        [Description("One full sentence stating the trap itself — this is the index line other agents scan.")] string title,
+        [Description("2-6 sentences: what happens, why it does not look like that, what to do instead.")] string body,
+        [Description("Repo-relative path or subsystem the finding applies to, e.g. 'src/ClaudeDo.Worker/Planning'.")] string scope = "",
+        [Description("List id or name. Optional when exactly one list has a working directory.")] string list = "",
+        CancellationToken cancellationToken = default)
+    {
+        var target = await _locator.ResolveForListAsync(list, cancellationToken);
+        var head = await GitHead.ShortAsync(target.WorkingDir, cancellationToken);
+
+        var outcome = await _store.SaveAsync(
+            target.WorkingDir,
+            new FindingInput(slug, title, body, scope, SourceTaskId: "", VerifiedAgainst: head),
+            cancellationToken,
+            target.Tracked);
+
+        return new SaveFindingResult(
+            true, outcome.Slug, target.ListName, outcome.Path, outcome.TotalFindings, outcome.NearCapacity);
+    }
+}