fix(MORG-48): sync without checking out branches

[developerdavi]

Closes MORG-48

Summary

• Step 1 (pull default branch): when not already on the default branch, uses git fetch origin main:main (via the existing fetchAndUpdateBranch helper) instead of checkout → pull → checkout. This updates the local branch ref without switching to it.
• Step 4 (rebase/merge): removes the checkout-away/back pattern. For non-current branches with a worktree, passes cwd: branch.worktreePath to rebaseBranch/mergeBranch so git operates in the right directory. For non-current branches without a worktree, skips with an informational message ("switch to it to update").
• rebaseBranch and mergeBranch in git/index.ts each gain an optional cwd parameter to enable the above.

What's unchanged

• The deletion case (Step 3) still checks out the default branch when the current branch is being deleted — this is the accepted exception per the acceptance criteria.

Test plan

• morg sync from a feature branch — should pull latest main without ever switching to main
• morg sync --all with multiple active branches that have worktrees — rebase/merge runs in each worktree's directory, user stays on current branch
• morg sync --all with a non-current branch that has no worktree — shows "switch to it to update" message and skips
• morg sync when current branch is deleted during sync — correctly checks out default branch (exception case)
• All 45 existing tests pass (pnpm test)