[codex] add source branch to TUI create form

[getsolaris]

Summary

• Add an optional Source branch field to the TUI worktree create flow.
• Preserve blank-input fallback to the configured base or origin/main, while passing typed refs into createWorktreeFlow.
• Split the create view into smaller controller/content/summary helpers and add fetch-ref safety guards.

Validation

• bun run typecheck
• bun test src/tui/create-worktree-input.test.ts src/core/git.test.ts (50 pass)
• bun test (355 pass)
• bun run build
• TypeScript LSP diagnostics on changed files: no errors
• Manual TUI QA covered typed source branch and blank default source behavior before publish