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/backend/__init__.py

@@ -53,7 +53,6 @@ class BottleSpec:
     agent_name: str
     copy_cwd: bool
     user_cwd: str
-    forward_oauth_token: bool
 
 
 @dataclass(frozen=True)