fix: Validate sync-branch at config-time and runtime (closes #1166) (#1168)

* fix(config): validate sync-branch at config time

Add sync-branch validation to validateYamlConfigValue() to reject
main/master at config time, preventing the validation bypass in GH#1166.

- Add case for sync-branch and sync.branch keys
- Inline validation logic to avoid import cycle (config <-> syncbranch)
- Add unit tests for rejection (main/master) and acceptance (valid names)

Part of: #1166

* fix(sync): add runtime guard for sync-branch == current-branch

Add dynamic runtime check before worktree operations to catch cases
where sync-branch matches the current branch. This provides defense
in depth for manual YAML edits, pre-fix configs, or non-main/master
branch names (trunk, develop, production, etc.).

- Check IsSyncBranchSameAsCurrent() after hasSyncBranchConfig is set
- Position check BEFORE worktree entry (CWD changes inside worktree)
- Add integration test TestSync_FailsWhenOnSyncBranch

Part of: #1166

* docs: note main/master restriction in sync-branch FAQ

Clarifies that git worktrees cannot checkout the same branch in
multiple locations, so main/master cannot be used as sync branch.

docs/PROTECTED_BRANCHES.md

@@ -426,7 +426,7 @@ No! This is a pure git solution that works on any platform. Just protect your `m
 
 ### Can I use a different branch name?
 
-Yes! Use any branch name you want:
+Yes! Use any branch name except `main` or `master` (git worktrees cannot checkout the same branch in multiple locations):
 
 ```bash
 bd init --branch my-custom-branch