feat: add --comments-file to pr review for inline comments (task 0035)

`pr review <n>` gains `--comments-file <path>`, a JSON array of inline
comments submitted with the review. Each entry is one of two exclusive
shapes: a new comment `{ path, line, body }` (mapped to `new_position`,
always the new side) or a reply `{ reply_to, body }`. A reply carries no
line or side — gitea-axi finds the target via the reviews-plus-comments
fan-out (there is no get-comment-by-id endpoint), reconstructs its anchor
from the target's own `diff_hunk`, and infers old/new side from it, so a
same-line post threads with the existing conversation. All entries map
onto the review-submission payload's `comments[]`; no new HTTP layer is
added. An unknown `reply_to` is a VALIDATION_ERROR raised before the POST,
and the submitted inline-comment count rides the action block.

The shared path-resolve-and-read behind --body-file and --comments-file is
extracted into src/flag-file.ts.

src/diff.ts

@@ -1,6 +1,6 @@
 import type { GiteaClient } from "./client.js";
 import type { RepoContext } from "./context.js";
-import { classifyHttpError } from "./errors.js";
+import { axiError, classifyHttpError } from "./errors.js";
 
 /** The raw-diff truncation limit, distinct from the body/comment limits. */
 export const DIFF_TRUNCATE_LIMIT = 4000;
@@ -73,3 +73,56 @@ export function trimDiffHunk(hunk: string): string {
   }
   return [lines[0], ...lines.slice(-2)].join("\n");
 }
+
+/**
+ * The file-line anchor of a review comment, as the side-tagged position the
+ * review-submission payload wants: exactly one of `new_position` (new side) or
+ * `old_position` (old side) is set.
+ */
+export interface HunkAnchor {
+  new_position?: number;
+  old_position?: number;
+}
+
+const HUNK_HEADER = /^@@ -(\d+)(?:,\d+)? \+(\d+)(?:,\d+)? @@/;
+
+/**
+ * Reconstruct the file-line anchor of the line a review comment's `diff_hunk`
+ * belongs to. Gitea builds a comment's `diff_hunk` so it ends at the commented
+ * line, so the anchor is that last body line: starting from the `@@ -old +new @@`
+ * header, walk the hunk tracking old- and new-file line numbers, and read the
+ * last line off. An added (`+`) or context (` `) line anchors on the new side
+ * (`new_position`); a deleted (`-`) line anchors on the old side (`old_position`).
+ *
+ * This lets a reply post a matching inline comment that threads with its target
+ * without the caller supplying any line or side — Gitea joins comments on the
+ * same line into one conversation.
+ */
+export function anchorFromDiffHunk(hunk: string): HunkAnchor {
+  const lines = hunk.split("\n");
+  const header = HUNK_HEADER.exec(lines[0] ?? "");
+  if (header === null) {
+    throw axiError(
+      "Gitea returned a review comment whose diff_hunk has no @@ header to anchor a reply",
+      "UNKNOWN",
+    );
+  }
+  let oldLine = Number(header[1]);
+  let newLine = Number(header[2]);
+  // An empty body is degenerate; default to the header's new-side start.
+  let anchor: HunkAnchor = { new_position: newLine };
+  for (const line of lines.slice(1)) {
+    if (line.startsWith("-")) {
+      anchor = { old_position: oldLine };
+      oldLine += 1;
+    } else if (line.startsWith("+")) {
+      anchor = { new_position: newLine };
+      newLine += 1;
+    } else {
+      anchor = { new_position: newLine };
+      oldLine += 1;
+      newLine += 1;
+    }
+  }
+  return anchor;
+}