feat(walkthrough): guided AI walkthrough for diffs, branches, and PRs (#2572)

A diff is ordered by file path, which is almost never the order in which a
change makes sense. This adds a Walkthrough surface that reorders it: the model
groups related hunks into stops, explains what each group changes about
behavior, and orders the stops so each builds on the last. It explains and
orders; judging code stays with the existing Review action.

Reviews uncommitted work (all, staged, unstaged), a branch against its base, or
a pull request. Generation is always user-initiated — nothing runs on a timer,
on a file change, or as a side effect of opening a panel.

Invariants worth preserving:

- Hunk identity is derived on the server and only there. Ids are content
  hashes, so an anchor that no longer resolves is proof the code it described
  changed, and staleness needs no heuristics. The client matches ids to ids and
  never recomputes them; two implementations would have to agree forever.
- The digest is never truncated. A diff that does not fit the model's context
  is refused with an actionable reason, because a walkthrough written against
  half a diff reads as confident and is wrong.
- Nothing disappears. Lockfiles and other generated output are excluded from
  the model's input by name — never by size — and everything no stop covers is
  listed at the end, so "have I seen all of it" stays answerable.
- Cost is explicit. Results are content-addressed, so returning the working
  tree to an earlier state costs nothing; generation outlives its request, so a
  refresh detaches the client rather than discarding paid-for work, and only an
  explicit cancel stops it.

Supporting changes to shared modules:

- git: expose the existing getRangeDiff as GET /api/git
  listUntrackedPaths and getUntrackedDiffs. The latter resolve the repository
  once for a batch instead of per file, taking a panel
  ~340ms on an 80-file working tree.
- small-model: structured output across four wire forma
  and abort signal, and an onOverflow policy so an oversized prompt fails
  loudly instead of being silently clipped. A provider
  remembered so the prompt-side fallback goes first next time.
- models.dev metadata: surface structured_output as tri
  false blocks a model, a missing field does not, because the catalog omits it
  for roughly half of all models.

Desktop and tablet only: VS Code serves Git through its
these routes, and the mobile shell does not consume the surface registry.

Docs: packages/docs walkthrough page in English and all eight locales.

packages/web/server/lib/git/service.js

@@ -488,6 +488,16 @@ const createRepositoryGitContext = async (directory) => {
   return { directoryPath, directoryGit, repoRoot, git };
 };
 
+/**
+ * Absolute repository root for a directory anywhere inside it. Callers that key
+ * persisted data by repository need this so two directories in the same
+ * repository do not address different records.
+ */
+export async function getRepositoryRoot(directory) {
+  const { repoRoot } = await createRepositoryGitContext(directory);
+  return repoRoot;
+}
+
 const resolveGitInternalPath = async (repoRoot, git, gitPath) => {
   const resolved = await git.raw(['rev-parse', '--git-path', gitPath]);
   return path.resolve(repoRoot, resolved.trim());
@@ -2427,6 +2437,78 @@ export async function getDiff(directory, { path: filePath, staged = false, conte
   }
 }
 
+/**
+ * Individual untracked file paths, honoring ignore rules.
+ *
+ * Deliberately not `--directory`: collapsed directory entries end in a slash
+ * and are not valid inputs to the per-file diff helpers, so a caller would
+ * silently lose every file inside a new directory. Listing files costs more
+ * entries but each one is usable.
+ *
+ * Callers that only need this list should not pay for `getStatus`, which also
+ * computes ahead/behind, diff stats, and merge state — an order of magnitude
+ * more work for an answer they throw away.
+ */
+export async function listUntrackedPaths(directory) {
+  const { repoRoot } = await createRepositoryGitContext(directory);
+  const result = await runGitCommand(repoRoot, [
+    'ls-files',
+    '--others',
+    '--exclude-standard',
+  ]);
+  if (!result.success) return [];
+  return String(result.stdout || '')
+    .split('\n')
+    .map((line) => line.trim())
+    .filter(Boolean);
+}
+
+/**
+ * Diffs for untracked files, produced against an empty tree.
+ *
+ * `getDiff` re-resolves the repository context on every call, which costs an
+ * extra `rev-parse` per file; a walkthrough of a branch with thirty new files
+ * pays that thirty times. This resolves once and reuses it, with a bounded pool
+ * so a repository full of new files cannot flood the process table.
+ *
+ * Returns one entry per input path, in order; unreadable paths yield `''`
+ * rather than failing the batch.
+ */
+export async function getUntrackedDiffs(directory, filePaths = [], { concurrency = 8, contextLines = 3 } = {}) {
+  const paths = (Array.isArray(filePaths) ? filePaths : []).filter((value) => typeof value === 'string' && value);
+  if (paths.length === 0) return [];
+
+  const { directoryPath, directoryGit, repoRoot, git } = await createRepositoryGitContext(directory);
+  const results = new Array(paths.length).fill('');
+  let cursor = 0;
+
+  const worker = async () => {
+    while (cursor < paths.length) {
+      const index = cursor++;
+      try {
+        const fileContext = await resolveGitFileContext(directoryPath, directoryGit, paths[index], repoRoot);
+        const args = ['diff', '--no-color'];
+        if (typeof contextLines === 'number' && !Number.isNaN(contextLines)) {
+          args.push(`-U${Math.max(0, contextLines)}`);
+        }
+        args.push('--no-index', '--', '/dev/null', fileContext.repoPath);
+        try {
+          results[index] = await git.raw(args);
+        } catch (error) {
+          // `git diff --no-index` exits 1 whenever there are differences, which
+          // for a new file is always.
+          results[index] = error?.exitCode === 1 && error?.message ? error.message : '';
+        }
+      } catch {
+        results[index] = '';
+      }
+    }
+  };
+
+  await Promise.all(Array.from({ length: Math.min(concurrency, paths.length) }, worker));
+  return results;
+}
+
 export async function getRangeDiff(directory, { base, head, path: filePath, contextLines = 3 } = {}) {
   const { directoryPath, directoryGit, repoRoot, git } = await createRepositoryGitContext(directory);
   const baseRef = typeof base === 'string' ? base.trim() : '';