feat(external-tools): add cross-platform external tool discovery

Adds a new external tools subsystem to detect and launch
diff and editor tools across Windows, macOS, and Linux.
It exposes data models for tools, commands, and results to the UI
and serializes them for consumption by the app.

- Implement cross-platform discovery of editors and diff tools
- Expose serialized results to the UI for user selection
- Centralize per-OS known tool lists and overrides

src/lib/components/HistoryPanel.svelte

@@ -1,5 +1,5 @@
 <script lang="ts">
-  import { Cherry, ChevronDown, ChevronRight, EllipsisVertical, GitBranch, GitMerge, LoaderCircle, RotateCcw, X } from "@lucide/svelte";
+  import { Cherry, ChevronDown, ChevronRight, EllipsisVertical, GitBranch, GitMerge, LoaderCircle, RotateCcw, StickyNote, X } from "@lucide/svelte";
   import type { FileStatusKind, GitCommit, GitCommitFile } from "../types";
 
   interface GraphSegment {
@@ -52,6 +52,7 @@
     onCreateBranchFromCommit: (commit: GitCommit) => void;
     onCherryPickCommit: (commit: GitCommit) => void;
     onRevertCommit: (commit: GitCommit) => void;
+    onOpenCommitNote: (commit: GitCommit) => void;
     onSelectCommit: (commit: GitCommit) => void;
   }
 
@@ -75,6 +76,7 @@
     onCreateBranchFromCommit = () => {},
     onCherryPickCommit = () => {},
     onRevertCommit = () => {},
+    onOpenCommitNote = () => {},
     onSelectCommit = () => {},
   }: Props = $props();
 
@@ -445,6 +447,20 @@
     await onRevertCommit(commit);
   }
 
+  async function openContextCommitNote() {
+    const commit = contextCommit;
+    if (!commit || isBusy) return;
+    closeCommitContextMenu();
+    onSelectCommit(commit);
+    await onOpenCommitNote(commit);
+  }
+
+  async function openCommitNote(commit: GitCommit) {
+    if (isBusy) return;
+    onSelectCommit(commit);
+    await onOpenCommitNote(commit);
+  }
+
   function handleWindowKeydown(event: KeyboardEvent) {
     if (event.key !== "Escape") return;
     closeCommitContextMenu();
@@ -720,6 +736,16 @@
             <div class="commit-actions">
               <time class="commit-time" datetime={item.date}>{formatCommitDate(item.date)}</time>
               <div class="commit-action-buttons">
+                <button
+                  class="commit-menu-button commit-note-button"
+                  type="button"
+                  onclick={() => openCommitNote(item)}
+                  disabled={isBusy}
+                  title={`Open internal note for ${item.short_hash}`}
+                  aria-label={`Open internal note for ${item.short_hash}`}
+                >
+                  <StickyNote size={14} aria-hidden="true" />
+                </button>
                 <button
                   class="commit-menu-button"
                   type="button"
@@ -767,6 +793,10 @@
         <GitBranch size={14} aria-hidden="true" />
         Branch
       </button>
+      <button type="button" role="menuitem" onclick={openContextCommitNote} disabled={isBusy}>
+        <StickyNote size={14} aria-hidden="true" />
+        Note
+      </button>
       <button type="button" role="menuitem" onclick={restoreContextCommit} disabled={isBusy}>
         <RotateCcw size={14} aria-hidden="true" />
         Restore