docs(04): add code review fix report

.planning/phases/04-auth-passthrough/04-REVIEW-FIX.md

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+---
+phase: 04-auth-passthrough
+fixed_at: 2026-04-10T00:00:00Z
+review_path: .planning/phases/04-auth-passthrough/04-REVIEW.md
+iteration: 1
+findings_in_scope: 4
+fixed: 4
+skipped: 0
+status: all_fixed
+---
+
+# Phase 04: Code Review Fix Report
+
+**Fixed at:** 2026-04-10
+**Source review:** .planning/phases/04-auth-passthrough/04-REVIEW.md
+**Iteration:** 1
+
+**Summary:**
+- Findings in scope: 4 (2 Critical, 2 Warning; Info excluded by fix_scope)
+- Fixed: 4
+- Skipped: 0
+
+## Fixed Issues
+
+### CR-01: CREDS_FILE path resolves against the wrong directory on the host
+
+**Files modified:** `claudebox.sh`
+**Commit:** adb9dd1
+**Applied fix:** Changed `CREDS_FILE` from `$HOME/.claude/.credentials.json` to `$HOME/.claudebox/.credentials.json`. Added comments explaining that the `~/.claude -> ~/.claudebox` symlink only exists inside the sandbox at runtime, so host-side credential detection must use the real claudebox config directory path.
+
+---
+
+### CR-02: Credentials bind target uses a symlink path — may silently fail
+
+**Files modified:** `claudebox.sh`
+**Commit:** adb9dd1
+**Applied fix:** Changed the `BWRAP_ARGS` credential bind destination from `$HOME/.claude/.credentials.json` (symlink path, unresolvable by bwrap at bind time) to `$HOME/.claudebox/.credentials.json` (canonical real directory). Also updated the `--dry-run` output block to print the corrected destination path.
+
+---
+
+### WR-01: Credentials file mounted read-write instead of read-only
+
+**Files modified:** `claudebox.sh`
+**Commit:** adb9dd1
+**Applied fix:** Changed `--bind` to `--ro-bind` for the credentials bind in both the `BWRAP_ARGS` array (line ~366) and the `--dry-run` output block (line ~333). Also updated the `print_audit` mounts display to show `(read-only)` and display `$CREDS_FILE` (the actual host source path) instead of the hardcoded symlink path inside the sandbox.
+
+---
+
+### WR-02: dry-run ENV_ARGS loop hard-codes stride of 3 — breaks if any non-setenv arg is added
+
+**Files modified:** `claudebox.sh`
+**Commit:** 0922b75
+**Applied fix:** Added a guard assertion before the stride-3 loop that checks `${#ENV_ARGS[@]} % 3 != 0` and exits with a `BUG:` message if the invariant is violated. This catches any future breakage immediately rather than producing silently mangled output. Also changed `(( dry_run_i += 3 ))` to `dry_run_i=$(( dry_run_i + 3 ))` to use safe arithmetic assignment compatible with `set -euo pipefail` (which `writeShellApplication` enables).
+
+---
+
+_Fixed: 2026-04-10_
+_Fixer: Claude (gsd-code-fixer)_
+_Iteration: 1_