#758 fix: guard worktrunk init and globals

Changes\n- Guard worktrunk shell init across fish/bash/zsh\n- Ensure rustup stable toolchain for cargo globals\n- Export BUN_INSTALL via home session vars\n\n## Technical Details\n- Move worktrunk init to fish interactive shell after PATH setup\n- Run rustup toolchain install/default in cargo globals activation\n- Set BUN_INSTALL for consistent bun global paths\n\n## Testing\n- make build && make switch\n\nGenerated with GitHub Copilot CLI by GPT-5

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Summary by cubic

Guarded Worktrunk shell initialization and hardened global toolchains to prevent shell errors and ensure consistent paths. Adds rustup stable toolchain for cargo globals and sets BUN_INSTALL for predictable bun installs.

• Bug Fixes
  • bash/zsh: only run Worktrunk init if wt exists.
  • fish: run Worktrunk init after PATH setup and guard with type check.
  • cargo globals: add rustc to PATH; install and set rustup stable before activation.
  • npm globals: export BUN_INSTALL="$HOME/.bun" for consistent bun global paths.