[bug] update: exits 0 after the upgrade actually failed #242

Environment

• CLI: bl 2.0.1 at the time of failure (Skill: 2.0.1)
• Node: v22.23.3
• OS: Linux 5.15.0-198-generic x86_64 (Ubuntu)
• Install channel: npm global (npm prefix -g → /home/lin/.npm-global)
• Region: cn (domestic)

Reproduce

1. Install bl globally via npm.
2. Make the npm global prefix read-only (here: a read-only filesystem sandbox).
3. With a newer version available (2.0.1 → 2.1.0), run:

bl update; echo "exit=$?"

Expected

A failed upgrade exits non-zero.

Actual

The upgrade failed with EROFS, the CLI printed Automatic update failed. Please run manually: — and still exited 0.

Any driver that checks only the exit code (CI, cron, agent) will record the
upgrade as successful while the old version is still installed. In this case the
caller believed 2.0.1 was the latest until it re-read bl --version.

Full output

Current version: 2.0.1
Install method: npm
Checking for updates...
Latest version: 2.1.0

Updating bailian-cli via npm...

npm error code EROFS
npm error syscall rename
npm error path /home/lin/.npm-global/lib/node_modules/bailian-cli
npm error dest /home/lin/.npm-global/lib/node_modules/.bailian-cli-RC0fMVNr
npm error errno -30
npm error rofs EROFS: read-only file system, rename '/home/lin/.npm-global/lib/node_modules/bailian-cli' -> '/home/lin/.npm-global/lib/node_modules/.bailian-cli-RC0fMVNr'
npm error rofs Often virtualized file systems, or other file systems
npm error rofs that don't support symlinks, give this error.
npm error A complete log of this run can be found in: <npm debug log>

Automatic update failed. Please run manually:
  npm install -g bailian-cli@latest
exit=0

Already tried

• Confirmed the exit code with both echo $? and ${PIPESTATUS[0]} — both 0
• The failure is not shell-related: bl update itself returns 0
• Same behaviour regardless of whether npm's cache/logs directories are writable
• After granting write access, the retry succeeded (2.0.1 → 2.1.0) and printed
  the skill-update result normally — so only the failure branch reports success

Notes

• Frequency: always on the failure path
• Related: [bug] --config <profile> silently ignores stored credentials; exits 0 unauthenticated #229 reports --config <profile> also exiting 0 on failure
  (exits 0 unauthenticated). Same class of defect: the process exit code does
  not reflect the outcome, and the CLI's own documented contract treats exit 0 as
  SUCCESS.
• Suggested fix: propagate npm's exit code from the bl update failure branch
  (or return a dedicated non-zero code), so bl update && ... and agent drivers
  can detect the failure.