fix(desktop): keep installed skilllite when prebuild has no engine

[cursor]

Summary

• Standalone desktop prebuild deleted ~/.skilllite/bin/skilllite before using that same file as the fallback binary. A tauri build / npm run prebuild:tauri with no engine checkout removed the user's CLI and then failed.
• Lookup and copy now happen before any delete. If the only installed binary is already ~/.skilllite/bin/skilllite, it is left in place. A failed copy does not remove the previous destination.

Task Linkage

• Task ID: N/A
• Task folder: lightweight bugfix; no tasks/TASK-.../ folder

Injected Specs

• spec/verification-integrity.md
• spec/testing-policy.md
• spec/docs-sync.md (reviewed; no doc change)
• spec/architecture-boundaries.md (not applicable)
• spec/security-nonnegotiables.md (not applicable)

Validation Evidence

• Commands executed:
  • bash skilllite-assistant/scripts/test-engine-root.sh
  • Replayed the old delete-then-lookup order against HEAD:skilllite-assistant/scripts/prebuild-skilllite.sh and a throwaway HOME
• Key results:
  • engine-root.sh checks passed (exit 0). The missing-source case prints cp: cannot stat ... does-not-exist and still leaves the destination intact.
  • Old script: lookup_line=15, rm_line=13, order assertion FAIL.
  • After rm -f ~/.skilllite/bin/skilllite, skilllite_find_installed_bin reports LOOKUP_AFTER_DELETE: not found.
• Not run: cargo fmt --check, cargo clippy, cargo test. This change is shell-only under skilllite-assistant/scripts/.

Regression Scope

• Areas likely affected:
  • skilllite-assistant/scripts/prebuild-skilllite.sh (beforeBuildCommand)
  • skilllite-assistant/scripts/prebuild-skilllite-dev.sh (forced dev refresh with no engine checkout)
• Explicit non-goals:
  • Engine CLI/MCP behavior, sandbox policy, and the engine-checkout cargo install path (that path still replaces ~/.skilllite/bin after an engine root is found)

Docs Sync (EN/ZH)

• Not needed
• Files: none. The script header already described reusing an installed binary; this restores that behavior. No new env var or command.

Review Checklist

• Lightweight mode (Task ID: N/A); no task-folder acceptance file
• tasks/.../STATUS.md not used
• tasks/.../REVIEW.md not used
• tasks/board.md not updated (no task folder)