fix(#1013): resolve worktree.baseRef from user/global settings cascade (#1038)

* fix(#1013): resolve worktree.baseRef from user/global settings cascade

cmdWorktreeBaseCheck resolved worktree.baseRef from the project checkout's
.claude/ only (settings.local.json then settings.json). A user/global
worktree.baseRef:"head" — the layer /config writes and the harness honors,
and the only sensible place for a machine-wide preference — was invisible. On a
phase lane (HEAD ahead of origin/HEAD, or no origin/HEAD symref) base-check
returned shouldDegrade:true and execute-phase forced sequential execution,
silently losing the parallel worktree execution the user configured.
CLAUDE_CONFIG_DIR (relocated user config dir) was also ignored.

resolveEffectiveBaseRef now accepts an optional user/global config dir and reads
its settings.json as a third, lowest-precedence layer (project local > project
shared > user/global). cmdWorktreeBaseCheck resolves it via
getGlobalConfigDir('claude'), which honors CLAUDE_CONFIG_DIR. The existing
project-level reads stay as higher-precedence overrides and the injectable
readFile seam is preserved, keeping the unit tests hermetic.

Closes #1013

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* chore(#1013): backfill changeset PR number to 1038

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

---------

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

src/worktree-base-ref.cts

@@ -14,6 +14,7 @@ import fs from 'node:fs';
 import path from 'node:path';
 
 import { execGit as execGitSeam } from './shell-command-projection.cjs';
+import { getGlobalConfigDir } from './runtime-homes.cjs';
 
 // ─── Internal helpers ─────────────────────────────────────────────────────────
 
@@ -160,14 +161,21 @@ export function applyWorktreeBaseRef(settings: Record<string, unknown>): {
 }
 
 /**
- * Reads settings.local.json then settings.json under claudeDir, extracts
- * worktree.baseRef from the first file that provides a non-null string value.
+ * Reads settings files in a 3-layer cascade and extracts worktree.baseRef from
+ * the first layer that provides a non-null string value. Layers (highest to lowest
+ * precedence):
+ *   1. project local  — <claudeDir>/settings.local.json
+ *   2. project shared — <claudeDir>/settings.json
+ *   3. user/global    — <userClaudeDir>/settings.json  (only when userClaudeDir is
+ *                       provided AND resolves to a different path than claudeDir)
  *
  * deps.readFile(path) must return the file contents or null on any error.
+ * userClaudeDir is optional; when absent/null the user/global layer is skipped.
  */
 export function resolveEffectiveBaseRef(
   claudeDir: string,
-  deps?: { readFile?: (p: string) => string | null }
+  deps?: { readFile?: (p: string) => string | null },
+  userClaudeDir?: string | null
 ): string | null {
   const readFile: (p: string) => string | null = deps?.readFile ?? ((p: string) => {
     try {
@@ -191,28 +199,47 @@ export function resolveEffectiveBaseRef(
     }
   }
 
+  // Layer 1: project local
   const localRef = parseBaseRef(localPath);
   if (localRef !== null) return localRef;
 
-  return parseBaseRef(sharedPath);
+  // Layer 2: project shared
+  const sharedRef = parseBaseRef(sharedPath);
+  if (sharedRef !== null) return sharedRef;
+
+  // Layer 3: user/global (only when provided and not the same directory as claudeDir)
+  if (userClaudeDir && path.resolve(userClaudeDir) !== path.resolve(claudeDir)) {
+    const userSharedPath = path.join(userClaudeDir, 'settings.json');
+    const userRef = parseBaseRef(userSharedPath);
+    if (userRef !== null) return userRef;
+  }
+
+  return null;
 }
 
 /**
  * CLI command: check current worktree base-ref degradation status.
  *
- * Reads effective baseRef from <cwd>/.claude settings, runs degradation
- * evaluation, writes JSON result to stdout (or injected write), and returns
- * the result object.
+ * Reads effective baseRef from <cwd>/.claude settings (3-layer cascade:
+ * project local → project shared → user/global), runs degradation evaluation,
+ * writes JSON result to stdout (or injected write), and returns the result object.
+ *
+ * deps.userClaudeDir overrides the user/global config directory resolution
+ * (default: getGlobalConfigDir('claude'), which honours CLAUDE_CONFIG_DIR).
  */
 export function cmdWorktreeBaseCheck(
   cwd: string,
   _args: string[],
-  deps?: { execGit?: ExecGitFn; readFile?: (p: string) => string | null; write?: (s: string) => void }
+  deps?: { execGit?: ExecGitFn; readFile?: (p: string) => string | null; write?: (s: string) => void; userClaudeDir?: string | null }
 ): ReturnType<typeof evaluateWorktreeBaseDegrade> {
   const claudeDir = path.join(cwd, '.claude');
+  const userClaudeDir = Object.prototype.hasOwnProperty.call(deps ?? {}, 'userClaudeDir')
+    ? (deps as { userClaudeDir?: string | null }).userClaudeDir
+    : getGlobalConfigDir('claude');
   const effectiveBaseRef = resolveEffectiveBaseRef(
     claudeDir,
-    deps?.readFile ? { readFile: deps.readFile } : undefined
+    deps?.readFile ? { readFile: deps.readFile } : undefined,
+    userClaudeDir
   );
   const result = evaluateWorktreeBaseDegrade({
     cwd,