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/start.py

@@ -39,7 +39,8 @@ from ...bottle_state import (
 from ...image_cache import StaleImageError
 from ...log import info, die
 from ...manifest import Manifest, ManifestIndex
-from .._common import PROG, USER_CWD, read_tty_line
+from ..constants import PROG
+from ...util import read_tty_line
 from .. import tui
 
 
@@ -116,7 +117,7 @@ def cmd_start(argv: list[str]) -> int:
         # threading a no_cache field through every backend's plan dataclass.
         os.environ["BOT_BOTTLE_NO_CACHE"] = "1"
 
-    manifest = ManifestIndex.resolve(USER_CWD)
+    manifest = ManifestIndex.resolve(os.getcwd())
 
     if args.headless:
         return _start_headless(
@@ -167,7 +168,7 @@ def cmd_start(argv: list[str]) -> int:
         manifest=manifest,
         agent_name=agent_name,
         copy_cwd=args.cwd,
-        user_cwd=USER_CWD,
+        user_cwd=os.getcwd(),
         label=label,
         color=color,
         bottle_names=bottle_names,
@@ -235,7 +236,7 @@ def _start_headless(
         manifest=manifest,
         agent_name=agent_name,
         copy_cwd=args.cwd,
-        user_cwd=USER_CWD,
+        user_cwd=os.getcwd(),
         label=label,
         color=args.color or "",
         bottle_names=bottle_names,