fix: keep repeated launcher on existing bridge

[k9ight000]

Problem

Launching CodexPlusPlus a second time can acquire the existing-instance path, start a temporary helper, reinject the page, and then exit. The page is left pointing at the temporary helper, so conversation delete requests fail even though the delete controls remain visible.

Fix

• Keep the repeated-launcher path limited to activating the existing Codex window.
• Do not start another helper, reinject the renderer, or start another bridge watchdog.
• Add a regression assertion that the existing-instance path cannot recreate bridge components.

Validation

• cargo fmt --check --package codex-plus-launcher
• cargo test -p codex-plus-launcher --no-run
• cargo test -p codex-plus-core --test launcher -j 1 -- --skip app_paths_resolves_portable_current_link_to_directory_version — 74 passed

The skipped test requires Windows symbolic-link privilege and is unrelated to this change. The published diff contains only apps/codex-plus-launcher/src/main.rs and was scanned for credentials and local user paths.