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.

docs/prds/0010-cred-proxy.md

@@ -306,14 +306,16 @@ Why the agent can't reach the sidecar's environ:
   `CredProxyConfig`, `Bottle.cred_proxy: CredProxyConfig`. Parse
   + validate route shape, role enum, path uniqueness, singleton-
   role constraints.
-- **`claude_bottle/backend/docker/prepare.py`** — switch the
-  agent's OAuth handling: when a route claims the
-  `anthropic-base-url` role, write `ANTHROPIC_BASE_URL` (pointing
-  at the proxy) plus a non-secret placeholder for
+- **`claude_bottle/backend/docker/prepare.py`** — drop the
+  legacy `CLAUDE_BOTTLE_OAUTH_TOKEN` → `CLAUDE_CODE_OAUTH_TOKEN`
+  forward entirely. cred-proxy is the only path the Anthropic
+  OAuth token reaches the bottle. When a route claims the
+  `anthropic-base-url` role, write `ANTHROPIC_BASE_URL`
+  (pointing at the proxy) plus a non-secret placeholder for
   `CLAUDE_CODE_OAUTH_TOKEN` (claude-code refuses to start
   otherwise; the proxy strips & replaces on every request).
-  When no such route exists, fall back to the pre-PRD-0010 path
-  (forward `CLAUDE_BOTTLE_OAUTH_TOKEN` as `CLAUDE_CODE_OAUTH_TOKEN`).
+  Bottles that need claude-code to authenticate must declare
+  the route; there is no fallback.
 - **`claude_bottle/backend/docker/backend.py`** — instantiate
   `DockerCredProxy` alongside `DockerPipelockProxy` and
   `DockerGitGate`; thread its `prepare` / `start` / `stop`