fix(completion): fall back on invalid manifests

[Whxuan0701]

Description

Preserve shell completion when a compiled CLI manifest exists but cannot be used:

• return an explicit fallback signal for unreadable, malformed, or non-array manifests
• let the main entry point continue through full adapter discovery on that signal
• keep the fast path unchanged for valid manifests

Previously the fast path converted a parse failure into an empty completion list and exited successfully, hiding all completion candidates.

Related issue: N/A (independently identified)

Type of Change

• 🐛 Bug fix
• ✨ New feature
• 🌐 New site adapter
• 📝 Documentation
• ♻️ Refactor
• 🔧 CI / build / tooling

Checklist

• I ran the checks relevant to this PR
• I updated tests or docs if needed
• I included output or screenshots when useful

Documentation (if adding/modifying an adapter)

• Added doc page under docs/adapters/ (if new adapter)
• Updated docs/adapters/index.md table (if new adapter)
• Updated sidebar in docs/.vitepress/config.mts (if new adapter)
• Updated README.md / README.zh-CN.md when command discoverability changed
• Used positional args for the command primary subject unless a named flag is clearly better
• Normalized expected adapter failures to CliError subclasses instead of raw Error

Screenshots / Output

npm run typecheck
> tsc --noEmit

npm run build
✅ Manifest compiled: 1331 entries

npm test
Test Files  591 passed (591)
Tests       6739 passed | 1 skipped (6740)

Risks / Known Gaps

Fallback discovery is slower than the manifest fast path, but it runs only when a manifest is unusable. Valid-manifest completion behavior is unchanged.