refactor(cli): trim _common to a PROG constants leaf; inline os.getcwd()

_common had rotted into a junk drawer. Trim it to what actually
justifies a shared leaf module and rename for legibility:

  * REPO_DIR: deleted (dead — nothing imported it).
  * read_tty_line: dropped the pointless re-export; cleanup/start/init
    now import it straight from bot_bottle.util.
  * USER_CWD: deleted. It captured os.getcwd() at import time, but
    nothing chdirs and no test patched it, so it was equivalent to a
    live os.getcwd() — inlined at the six call sites.
  * PROG: kept, now the sole member. It's still a leaf (both the
    dispatcher and the commands it imports need it) so it can't move
    into __init__ without a circular import.

_common.py -> constants.py. Full unit suite green (2243); CLI dispatch
verified.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

bot_bottle/cli/__init__.py

@@ -11,7 +11,7 @@ from ..errors import MissingEnvVarError
 from ..log import Die, die, error
 from ..manifest import ManifestError
 from ..orchestrator.store.store_manager import StoreManager
-from ._common import PROG
+from .constants import PROG
 from .commands import list as _list_mod
 from .commands.backend import cmd_backend
 from .commands.cleanup import cmd_cleanup