Closes credential-scope bypass where an agent without a `secure_cli_agent_grants` row could still invoke a registered binary via shell fallthrough and pick up inherited env (`$GH_TOKEN`) or on-disk OAuth state. Registration is now an authorization boundary, not only a credential-injection hint.
- Add `SecureCLIStore.IsRegisteredBinary` on PG + SQLite backends — case-insensitive, tenant-scoped.
- New gate branch in `internal/tools/shell.go` (after normalization, before exec approval) with shell-wrapper unwrap depth 3 (sh/bash/zsh/dash -c, env K=V, nohup, stdbuf, timeout), 2s DB-lookup timeout, fail-CLOSED on error. New log events: `security.credentialed_binary_denied`, `security.credentialed_binary_gate_error`, `security.credentialed_binary_wrapper_too_deep`.
- Env scrubbing on host fall-through (`internal/tools/env_scrub.go`) strips static credential keys (GH_TOKEN, AWS_*, OPENAI_API_KEY, …) plus dynamic keys from tenant's registered binaries; preserves HOME/PATH/TERM/LANG/USER/TZ.
- Subagent ExecTool registration (`cmd/gateway_agents.go` → `buildSubagentToolsRegistry`) now receives the same `SecureCLIStore` — parent can't delegate to a child to bypass the gate.
- Binary names lowercased on Create/Update for symmetry with case-insensitive lookup.
- Thread `pgStores.SecureCLI` into `setupSubagents` in `cmd/gateway.go`.
- 25+ unit tests (gate, env-scrub, SQLite IsRegisteredBinary) + 5 PG integration tests (deny-ungranted, allow-granted, unregistered-unchanged, is_global-not-denied regression, shell-wrapper-bypass denied). All green.
