fix: create worktree when using --from and auto-create on switch

[dezren39]

Summary

• ez create --from now creates a worktree — Previously --from always skipped worktree creation due to !no_worktree && from.is_none(). The --from flag controls which branch is the parent, not whether a worktree is created.
• ez switch auto-creates worktrees for managed branches — Previously fell back to git checkout in the main worktree, hijacking it. Now creates a linked worktree for managed branches that don't have one.
• --no-worktree still works as an explicit opt-out, with or without --from.

Bug report

$ ez create local-timestamp --from main
✓ Created `local-timestamp` from `main`
# ... but "no worktree" in ez list

$ ez switch local-timestamp
✓ Switched to `local-timestamp`
# ... but checked out in main repo dir, not a worktree

Root cause

1. create.rs:98: let use_worktree = !no_worktree && from.is_none() — from.is_none() wrongly disabled worktree creation whenever --from was used.
2. checkout.rs:81-83: when a branch had no worktree, switch_to() did a raw git checkout instead of creating a worktree.

Changes

• src/cmd/create.rs — Removed from.is_none() from worktree condition, consolidated 3 code paths into 2 (worktree vs --no-worktree)
• src/cmd/checkout.rs — Added auto-worktree creation for managed branches without one on switch; trunk still does plain checkout
• 4 new tests covering both fixes and the --no-worktree opt-out

[rohoswagger]

looks good!

[rohoswagger]

@dezren39 could you fix up CI for this and then i can go ahead and merge it in :)