[kensai] deny excluded dirs in readFile fallback

Direct filesystem fallback in readFile bypassed the
WALK_EXCLUDE_DIRS guard, allowing reads into .git/.
Now checks the first path segment against the exclusion
set before falling through to stat.

Also fixes isBinary JSDoc to reflect extension-based
detection.

Author: xobotyi

claude/kensai/mcp/review/src/repofs/pathindex/pathindex.ts

@@ -9,7 +9,7 @@ import picomatch from "picomatch";
 import { ErrBinaryContent, LineIter } from "../lineiter/lineiter.ts";
 
 /** Directory names skipped during filesystem walk. Never descended into. */
-const WALK_EXCLUDE_DIRS: ReadonlySet<string> = new Set([".git"]);
+export const WALK_EXCLUDE_DIRS: ReadonlySet<string> = new Set([".git"]);
 
 /** Extensions treated as binary without opening the file. Stat-only — no line counting, no fd consumed. */
 const BINARY_EXTENSIONS: ReadonlySet<string> = new Set([
@@ -152,7 +152,7 @@ export type IndexEntryFile = {
 	lineCount: number;
 	/** Byte length of the longest line (excluding `\n`). 0 for binary/empty files. */
 	maxLineLen: number;
-	/** True when the file's leading bytes contain a null byte. */
+	/** Extension-based hint. Content-based detection (null byte probe) runs at read/scan time. */
 	isBinary: boolean;
 };
 

claude/kensai/mcp/review/src/repofs/repofs.ts

@@ -3,7 +3,7 @@ import path from "node:path";
 
 import { Repo } from "./git/git.ts";
 import type { FilterOptions } from "./pathindex/pathindex.ts";
-import { PathIndex } from "./pathindex/pathindex.ts";
+import { PathIndex, WALK_EXCLUDE_DIRS } from "./pathindex/pathindex.ts";
 import type { GrepOptions, GrepResult } from "./rg/rg.ts";
 import { grep } from "./rg/rg.ts";
 
@@ -92,6 +92,10 @@ export class RepoFs {
 		const absPath = path.resolve(this.#root, rel);
 
 		if (!entry) {
+			const firstSegment = rel.split("/")[0];
+			if (firstSegment && WALK_EXCLUDE_DIRS.has(firstSegment)) {
+				throw new RepoFsError(`file not found: ${rel}`);
+			}
 			const stat = await fsp.stat(absPath).catch(() => null);
 			if (!stat?.isFile()) {
 				throw new RepoFsError(`file not found: ${rel}`);