chore: restore backend/.env copy in worktree setup

[centdix]

Summary

• When .workmux.yaml was removed, the files.copy directive that copied backend/.env into new worktrees was lost
• Restores the copy in wm_copy_dependencies() in scripts/worktree-common.sh, which runs for both workmux and Claude hooks

Test plan

• Create a new worktree and verify backend/.env is copied from the main repo

🤖 Generated with Claude Code

[claude]

Claude finished @centdix's task in 1m 42s —— View job

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Review of PR #8452

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Feedback

LGTM — this is a clean, minimal fix.

The change correctly restores the backend/.env copy that was lost when .workmux.yaml was removed. It follows the exact same pattern as the existing copies in wm_copy_dependencies() (guard with -f/-d, cp, echo confirmation), and placing it first in the function makes sense since the backend env is needed before anything else runs.

No issues found — no bugs, no security concerns, no performance impact.

Reproduction instructions

This change affects the developer worktree setup workflow, not the app UI. To verify: create a new git worktree (e.g. git worktree add ../test-wt some-branch) with a backend/.env in the main repo, then confirm the file is copied into the new worktree's backend/ directory.

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