feat(cred_proxy)!: cred-proxy is the only Anthropic auth path

Removes the legacy `CLAUDE_BOTTLE_OAUTH_TOKEN` -> `CLAUDE_CODE_OAUTH_TOKEN`
forward in prepare.py. Bottles that need claude-code to authenticate
must declare a cred_proxy route with role: "anthropic-base-url" — there
is no fallback that hands the token to the agent directly.

Drops the now-dead BottleSpec.forward_oauth_token field, the CLI
setter that read CLAUDE_BOTTLE_OAUTH_TOKEN from the host env at
prepare time, and the forward_oauth_token=False arg in the six
pipelock integration tests.

PRD 0010 and README updated; the dev ~/claude-bottle.json gains an
anthropic-base-url route so the implementer/researcher agents keep
working.

BREAKING: bottles previously relying on the implicit OAuth forward
will now produce an agent environ without any Anthropic credential.
Verified with --dry-run: a bottle with no anthropic-base-url route
yields env_names: [] (no token at all); a bottle that declares the
route yields ANTHROPIC_BASE_URL plus a non-secret placeholder for
CLAUDE_CODE_OAUTH_TOKEN.

claude_bottle/cli/start.py

@@ -42,7 +42,6 @@ def cmd_start(argv: list[str]) -> int:
         agent_name=args.name,
         copy_cwd=args.cwd,
         user_cwd=USER_CWD,
-        forward_oauth_token=bool(os.environ.get("CLAUDE_BOTTLE_OAUTH_TOKEN")),
     )
 
     stage_dir = Path(tempfile.mkdtemp(prefix="claude-bottle-stage."))