fix: preserve nested paths in wt-rm and wt-destroy

[davidpoblador]

Summary

• Stop using basename on target argument in wt-rm and wt-destroy, which was truncating nested branch paths like fix/my-feature to just my-feature
• Add --force to git worktree remove in wt-destroy only, since it's the destructive variant that already deletes remote branches

Closes #38

Test plan

• just wt-add fix/test-nested then just wt-destroy fix/test-nested succeeds
• just wt-add simple then just wt-rm simple succeeds
• just wt-destroy main still refuses

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