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/info.py

@@ -3,10 +3,11 @@
 from __future__ import annotations
 
 import argparse
+import os
 
 from ...log import info
 from ...manifest import ManifestIndex
-from .._common import PROG, USER_CWD
+from ..constants import PROG
 
 
 def cmd_info(argv: list[str]) -> int:
@@ -14,7 +15,7 @@ def cmd_info(argv: list[str]) -> int:
     parser.add_argument("name", help="agent name defined in bot-bottle.json")
     args = parser.parse_args(argv)
 
-    names = ManifestIndex.resolve(USER_CWD)
+    names = ManifestIndex.resolve(os.getcwd())
     names.require_agent(args.name)
     manifest = names.load_for_agent(args.name)