feat: create tasks from existing branches

[TomCC7]

Summary

• reuse an exact-match local branch when creating a task worktree
• support the same behavior in the TUI and task create CLI
• preserve existing branch upstream configuration and skip unrelated base checks
• fail before tmux or database creation when the branch is already checked out elsewhere

Validation

• cargo fmt --all -- --check
• cargo test (416 tests passed)
• cargo clippy -- -D warnings