feat(cli): add launch selector TUI for start command (PRD 0051)

- Add bot_bottle/cli/tui.py: curses filter-select picker that opens
  /dev/tty directly so it works with redirected stdout/stdin
- Make `name` positional optional (nargs="?") in cmd_start; show agent
  picker when absent
- Show backend picker when no --backend flag and BOT_BOTTLE_BACKEND is
  unset; skip when either is explicit or the env var is present
- Add tests/unit/test_cli_tui.py covering _filter_items logic and
  short-circuit paths (empty list, unavailable tty)
- Add tests/unit/test_cli_start_selector.py covering all four dispatch
  combinations (both explicit, agent-absent, backend-absent, both-absent)
  and cancel semantics
- Activate PRD 0051

docs/prds/0051-launch-selector.md

@@ -1,6 +1,6 @@
 # PRD 0051: Launch selector
 
-- **Status:** Draft
+- **Status:** Active
 - **Author:** claude
 - **Created:** 2026-06-04
 - **Issue:** #185