Cleanup: BSMCP_INDEX_WORKER is dead doc — referenced in comments but not read

[Gumbees]

`BSMCP_INDEX_WORKER` shows up in doc comments on `crates/bsmcp-embedder/src/worker.rs` (post-#56 location) but nothing calls `env::var("BSMCP_INDEX_WORKER")`. It was a pre-split gate from when the worker lived inside bsmcp-server. After the v1.1.0 split and the #56 fold, the env var no longer does anything.

Fix

Drop the `BSMCP_INDEX_WORKER` mention from worker.rs's module-level doc. Also check any other comment / README references.

Trivial. Bundle with another small cleanup if convenient.