Add Git Worktree Explorer canvas (#2344)

* Add Git Worktree Explorer canvas

Add an interactive repository, worktree, branch, and commit graph with GitHub PR enrichment, safe inspection actions, and shared lane visualization.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 69dd5824-7094-4b82-8cfa-83c2fbe53307

* Potential fix for pull request finding

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

* Address review feedback for Git Worktree Explorer

Security
- Quote copied shell commands (git -C / git log) with shell-safe single quoting and --end-of-options via new public/shell-quote.mjs
- Move repository path into the untrusted data block of the commit Ask Copilot prompt

Performance
- Compute branch divergence with a single `git for-each-ref %(ahead-behind:...)` query (Git 2.41+), falling back to rev-list with bounded concurrency (8) instead of one process per branch
- Drop the unused `git show --stat` summary from commit details

Correctness
- Only attach same-repository pull requests to local branches (filter isCrossRepository / headRepositoryOwner)
- Resolve the default branch by full name or upstream instead of a suffix match (server + client)
- Preserve already-loaded commit pages when a load-more request fails; only reset requests show the error state
- Size the lane graph from the widest row (lanes + branch badges + text) and collapse >3 branch tips into a "+N more" badge

Accessibility / UX
- Add aria-pressed to view toggles and graph nodes; replace the unimplemented ARIA tree with group/button semantics
- preventDefault on Space/Enter for branch badges so keyboard activation does not scroll
- Use a lane palette that meets 3:1 contrast in both light and dark themes (tested)
- Mobile inspector no longer auto-opens on snapshot load and gains a close button

Tests: 19 -> 29

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 18350039-5e2b-40f0-b537-bc22cabcbb1b

* Migrate git-worktree-explorer to plugins/ manifest layout

Move the manifest from extensions/git-worktree-explorer/.github/plugin/plugin.json to plugins/git-worktree-explorer/plugin.json with README and copilot-extension.json, matching the extensions-container migration (#2334). Regenerated marketplace.json and docs.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 18350039-5e2b-40f0-b537-bc22cabcbb1b

* Address follow-up review: shell-specific quoting and sibling row controls

- quoteShellArg/formatShellCommand take a target shell; PowerShell doubles apostrophes while POSIX uses '\\''. app.js detects the platform and reports which syntax was copied.
- Commit rows no longer nest branch badge buttons inside the row button; the row button and badges are sibling controls.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 18350039-5e2b-40f0-b537-bc22cabcbb1b

* Make the closed mobile inspector inert and restore focus to the graph

When the inspector overlay is dismissed on narrow viewports it is now marked inert/aria-hidden so its controls leave the tab order and accessibility tree, focus returns to the selected graph control, Escape closes it, and viewport changes re-sync the state.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 18350039-5e2b-40f0-b537-bc22cabcbb1b

---------

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Aaron Powell <me@aaron-powell.com>
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>
Copilot-Session: 69dd5824-7094-4b82-8cfa-83c2fbe53307
Copilot-Session: 18350039-5e2b-40f0-b537-bc22cabcbb1b

extensions/git-worktree-explorer/public/graph-layout.mjs

@@ -0,0 +1,90 @@
+// Each color keeps at least 3:1 contrast against both the light (#ffffff)
+// and dark (#0d1117) canvas backgrounds so lanes stay traceable in either theme.
+const COLORS = [
+  "#0969da",
+  "#bf3989",
+  "#bf8700",
+  "#1a7f37",
+  "#8250df",
+  "#bc4c00",
+  "#1b7c83",
+  "#cf222e",
+];
+
+export const LANE_COLORS = COLORS;
+
+function nextColor(index) {
+  return COLORS[index % COLORS.length];
+}
+
+export function layoutCommitGraph(commits) {
+  let lanes = [];
+  let colorIndex = 0;
+  let maxLanes = 1;
+
+  const rows = commits.map((commit) => {
+    let laneIndex = lanes.findIndex((lane) => lane.sha === commit.sha);
+    if (laneIndex === -1) {
+      lanes.push({ sha: commit.sha, color: nextColor(colorIndex++) });
+      laneIndex = lanes.length - 1;
+    }
+
+    const before = lanes.map((lane) => ({ ...lane }));
+    const current = before[laneIndex];
+    const after = lanes.map((lane) => ({ ...lane }));
+    const firstParent = commit.parents[0] || null;
+
+    if (!firstParent) {
+      after.splice(laneIndex, 1);
+    } else {
+      const existingFirstParent = after.findIndex((lane, index) =>
+        index !== laneIndex && lane.sha === firstParent
+      );
+      if (existingFirstParent >= 0) {
+        after.splice(laneIndex, 1);
+      } else {
+        after[laneIndex] = { sha: firstParent, color: current.color };
+      }
+    }
+
+    for (const parent of commit.parents.slice(1)) {
+      if (after.some((lane) => lane.sha === parent)) continue;
+      const insertAt = Math.min(laneIndex + 1, after.length);
+      after.splice(insertAt, 0, { sha: parent, color: nextColor(colorIndex++) });
+    }
+
+    const transitions = [];
+    before.forEach((lane, index) => {
+      if (index === laneIndex) return;
+      const target = after.findIndex((candidate) => candidate.sha === lane.sha);
+      if (target >= 0) {
+        transitions.push({ from: index, to: target, color: lane.color, kind: "pass" });
+      }
+    });
+
+    commit.parents.forEach((parent, parentIndex) => {
+      const target = after.findIndex((lane) => lane.sha === parent);
+      if (target >= 0) {
+        transitions.push({
+          from: laneIndex,
+          to: target,
+          color: parentIndex === 0 ? current.color : after[target].color,
+          kind: parentIndex === 0 ? "first-parent" : "merge-parent",
+        });
+      }
+    });
+
+    maxLanes = Math.max(maxLanes, before.length, after.length);
+    lanes = after;
+    return {
+      commit,
+      laneIndex,
+      color: current.color,
+      transitions,
+      lanesBefore: before.length,
+      lanesAfter: after.length,
+    };
+  });
+
+  return { rows, maxLanes };
+}