Replace magic strings with StrEnum types for command flags

[vm-mishchenko]

Summary

This PR introduces type-safe enums for three command flags: branch_source, pr_mode, and worktree_mode. A new src/draft/types.py module defines BranchSource, PrMode, and WorktreeMode as StrEnum types. All command files are updated to use enum members instead of hardcoded string literals.

The change preserves backward compatibility—enum values match current strings exactly, so existing persisted state.json files continue to work without migration.

Test plan

• Verify JSON serialization: python -c "import json; from draft.types import BranchSource; print(json.dumps({'x': BranchSource.NEW}))" outputs {"x": "new"}
• Verify enum equality: BranchSource.NEW == "new" returns True; "existing" in (BranchSource.NEW, BranchSource.EXISTING) returns True
• Load existing state.json from before this change and verify RunContext.load followed by comparisons against enum members work correctly
• Run draft create on a new branch and verify state.json persists as "new", "open", "worktree" (raw strings)
• Run draft create --branch <existing> and verify correct string values in state.json
• Run draft babysit, draft fix-pr, draft continue on existing runs and verify same expected steps as before
• Verify no bare-string magic values remain in code (except protected literals: gh pr list --state open argument and git worktree subcommands)