Guard against main repo on target branch before worktree creation#

Problem#

When the main repo is checked out on the target feature branch, git branch -D silently fails, causing git worktree add -b to fatal with "branch already exists". This was the root cause of issue #246 being stuck for 12+ hours.

Task / Link#

Closes #266

Changes#

• Added a branch guard in WorktreeService.createWorktree() after git fetch origin
• Detects current branch via git rev-parse --abbrev-ref HEAD
• If the main repo is on the target branch, auto-switches to master with a warning log
• If git checkout master fails, throws a descriptive error (does not swallow)
• Detached HEAD state is treated as safe — no switch attempted
• Added two new unit tests: auto-switch success and checkout failure scenarios

Notes#

If the main repo has uncommitted changes when the conflict is detected, git checkout master will fail and a typed error is thrown — by design per issue scope (surface the error, do not stash).

Testing#

• 13 unit tests pass (11 existing + 2 new)
• New tests cover: auto-switch to master succeeds, and checkout failure throws descriptive error