Detect and auto-clear stale merge state

[alexrinass]

IsMergeInProgress() currently only checks whether .git/gitflow/state/merge.json exists. It doesn't validate whether the state is still meaningful. A stale state file causes confusing errors like unknown branch type: (see #87) and could block other operations.

How state becomes stale

• User manually resolves a merge outside of git-flow (git merge --abort, git checkout <branch>)
• Process crashes mid-operation (no conflict, just interrupted)
• State file is corrupted or has empty fields
• Related: Fix merge state not cleared when branch deletion fails during finish #97 (state not cleared when branch deletion fails)

Proposed solution

Add validation inside IsMergeInProgress() so all callers benefit automatically.

Checks:

1. Field validation -- critical fields must be non-empty (BranchType, FullBranchName, CurrentStep)
2. Conflict steps (merge, update_children) -- git must actually be in a merge or rebase state
3. Non-conflict steps (create_tag, delete_branch) -- the topic branch referenced in the state must still exist

If any check fails, clear the state file, print Note: Cleared stale merge state from a previous operation, and return false.