Fix bootstrap preflight repository detection

[zimakki]

Summary

• capture the preflight script repository path before entering Zsh functions
• use that captured path for no-argument guard and full preflight execution
• exercise no-argument canonical and linked-worktree behavior in the isolated suite

Why

Zsh changes $0 to the function name inside functions. The merged preflight therefore derived /Users/zimakki/code instead of the dotfiles checkout when executed without an explicit repository argument. This blocked the required post-merge migration.

Validation

• scripts/ci_checks.sh
• full preflight from the linked test checkout with the explicit test-only mutation override
• no-argument guard derives and rejects the actual linked worktree path