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/commands/init.py

@@ -11,7 +11,8 @@ from pathlib import Path
 from typing import Any
 
 from ...log import die, info, warn
-from .._common import PROG, USER_CWD, read_tty_line
+from ..constants import PROG
+from ...util import read_tty_line
 
 
 def cmd_init(argv: list[str]) -> int:
@@ -22,7 +23,7 @@ def cmd_init(argv: list[str]) -> int:
     if args.scope == "user":
         target_file = Path(os.environ["HOME"]) / "bot-bottle.json"
     else:
-        target_file = Path(USER_CWD) / "bot-bottle.json"
+        target_file = Path(os.getcwd()) / "bot-bottle.json"
 
     print(file=sys.stderr)
     info(f"bot-bottle init — adding a new agent to {target_file}")