fix(codex): relPath home containment — component boundary + platform case semantics

The warning-display helper matched home by lowercase string prefix, so
C:\Users\bob2\... rendered as inside ~ for home C:\Users\bob, and POSIX
case-sensitive paths were case-folded. Use path.relative() with a full
containment predicate (exact home, parent, cross-drive, sibling-prefix all
handled); case semantics now come from the platform path API.

devlog: 260715_cross_platform_audit/030

Author: lidge-jun

src/codex/project-config-warnings.ts

@@ -1,5 +1,5 @@
 import { existsSync, readFileSync } from "node:fs";
-import { dirname, join, resolve } from "node:path";
+import path, { dirname, join, resolve } from "node:path";
 import { expandUserPath } from "../config";
 import { defaultCodexHome } from "./home";
 import { readRootTomlString } from "./paths";
@@ -223,12 +223,24 @@ export function dedupeRelatedProjectCodexWarnings(
   });
 }
 
-function relPath(abs: string): string {
+/**
+ * Render a path under the user's home as `~/...` for warning display.
+ * Platform-correct containment (devlog 260715_cross_platform_audit/030): the old
+ * lowercase prefix match had no component boundary (`C:\Users\bob2` rendered as
+ * inside `~` for home `C:\Users\bob`) and case-folded on case-sensitive POSIX
+ * filesystems. `relative()` carries the right case semantics per platform; reject
+ * parent (`..`, `..\x`) and cross-drive (absolute) results.
+ */
+export function relPath(
+  abs: string,
+  pathApi: Pick<typeof path, "relative" | "sep" | "isAbsolute"> = path,
+): string {
   const home = process.env.USERPROFILE ?? process.env.HOME ?? "";
-  if (home && abs.toLowerCase().startsWith(home.toLowerCase())) {
-    return `~${abs.slice(home.length).replace(/\\/g, "/")}`;
-  }
-  return abs;
+  if (!home) return abs;
+  const rel = pathApi.relative(home, abs);
+  if (rel === "") return "~";
+  if (rel === ".." || rel.startsWith(`..${pathApi.sep}`) || pathApi.isAbsolute(rel)) return abs;
+  return `~/${rel.replace(/\\/g, "/")}`;
 }
 
 export function discoverProjectCodexConfigPaths(options: {

tests/project-config-warnings.test.ts

@@ -1,17 +1,69 @@
 import { afterEach, beforeEach, describe, expect, test } from "bun:test";
 import { mkdirSync, rmSync, writeFileSync } from "node:fs";
 import { tmpdir } from "node:os";
-import { join } from "node:path";
+import { join, posix, win32 } from "node:path";
 import {
   analyzeProjectCodexConfig,
   collectProjectCodexConfigWarnings,
   getCachedProjectConfigDiagnostics,
   isGlobalOpencodexRoutingActive,
   invalidateProjectConfigDiagnosticsCache,
   parseTrustedProjectPathsFromCodexConfig,
+  relPath,
   resolveEffectiveProjectModelProvider,
 } from "../src/codex/project-config-warnings";
 
+describe("relPath home containment (devlog 260715_cross_platform_audit/030)", () => {
+  let savedUserProfile: string | undefined;
+  let savedHome: string | undefined;
+
+  beforeEach(() => {
+    savedUserProfile = process.env.USERPROFILE;
+    savedHome = process.env.HOME;
+  });
+
+  afterEach(() => {
+    if (savedUserProfile === undefined) delete process.env.USERPROFILE; else process.env.USERPROFILE = savedUserProfile;
+    if (savedHome === undefined) delete process.env.HOME; else process.env.HOME = savedHome;
+  });
+
+  function setHome(value: string) {
+    process.env.USERPROFILE = value;
+    delete process.env.HOME;
+  }
+
+  test("win32: contained descendants render as ~/, exact home as ~", () => {
+    setHome("C:\\Users\\bob");
+    expect(relPath("C:\\Users\\bob\\proj\\.codex\\config.toml", win32)).toBe("~/proj/.codex/config.toml");
+    expect(relPath("C:\\Users\\bob", win32)).toBe("~");
+    // relative() case-folds on win32 (drive letters and components).
+    expect(relPath("c:\\users\\bob\\x", win32)).toBe("~/x");
+  });
+
+  test("win32: sibling prefix (bob vs bob2) is NOT rendered as home", () => {
+    setHome("C:\\Users\\bob");
+    expect(relPath("C:\\Users\\bob2\\proj\\config.toml", win32)).toBe("C:\\Users\\bob2\\proj\\config.toml");
+  });
+
+  test("win32: parent and cross-drive paths stay absolute", () => {
+    setHome("C:\\Users\\bob");
+    expect(relPath("C:\\Users", win32)).toBe("C:\\Users");
+    expect(relPath("D:\\work\\config.toml", win32)).toBe("D:\\work\\config.toml");
+  });
+
+  test("posix: comparison is case-sensitive (no false ~ for different-case home)", () => {
+    setHome("/home/Bob");
+    expect(relPath("/home/bob/x", posix)).toBe("/home/bob/x");
+    expect(relPath("/home/Bob/x", posix)).toBe("~/x");
+  });
+
+  test("no home env leaves paths untouched", () => {
+    delete process.env.USERPROFILE;
+    delete process.env.HOME;
+    expect(relPath("/anywhere/x", posix)).toBe("/anywhere/x");
+  });
+});
+
 let testDir = "";
 let previousHome: string | undefined;
 let previousCodexHome: string | undefined;