#23811: Config: fail closed when exec host=sandbox but sandbox mode is off

## Summary Describe the problem and fix in 2–5 bullets: - Problem: - Why it matters: - What changed: - What did NOT change (scope boundary): ## Change Type (select all) - [ ] Bug fix - [ ] Feature - [ ] Refactor - [ ] Docs - [ ] Security hardening - [ ] Chore/infra ## Scope (select all touched areas) - [ ] Gateway / orchestration - [ ] Skills / tool execution - [ ] Auth / tokens - [ ] Memory / storage - [ ] Integrations - [ ] API / contracts - [ ] UI / DX - [ ] CI/CD / infra ## Linked Issue/PR - Closes # - Related # ## User-visible / Behavior Changes List user-visible changes (including defaults/config). If none, write `None`. ## Security Impact (required) - New permissions/capabilities? (`Yes/No`) - Secrets/tokens handling changed? (`Yes/No`) - New/changed network calls? (`Yes/No`) - Command/tool execution surface changed? (`Yes/No`) - Data access scope changed? (`Yes/No`) - If any `Yes`, explain risk + mitigation: ## Repro + Verification ### Environment - OS: - Runtime/container: - Model/provider: - Integration/channel (if any): - Relevant config (redacted): ### Steps 1. 2. 3. ### Expected - ### Actual - ## Evidence Attach at least one: - [ ] Failing test/log before + passing after - [ ] Trace/log snippets - [ ] Screenshot/recording - [ ] Perf numbers (if relevant) ## Human Verification (required) What you personally verified (not just CI), and how: - Verified scenarios: - Edge cases checked: - What you did **not** verify: ## Compatibility / Migration - Backward compatible? (`Yes/No`) - Config/env changes? (`Yes/No`) - Migration needed? (`Yes/No`) - If yes, exact upgrade steps: ## Failure Recovery (if this breaks) - How to disable/revert this change quickly: - Files/config to restore: - Known bad symptoms reviewers should watch for: ## Risks and Mitigations List only real risks for this PR. Add/remove entries as needed. If none, write `None`. - Risk: - Mitigation: <!-- greptile_comment --> <h3>Greptile Summary</h3> Added config-time validation to fail closed when `tools.exec.host="sandbox"` is configured but sandbox mode is `"off"`. This prevents runtime errors by catching the misconfiguration early during config validation. - Validates both global `tools.exec.host` and per-agent `agents.list[].tools.exec.host` settings - Correctly defaults sandbox mode to `"off"` when not explicitly configured - Uses fallback logic for per-agent validation (agent-specific mode falls back to default mode) - Test coverage includes global rejection, global acceptance, and per-agent rejection scenarios <h3>Confidence Score: 5/5</h3> - This PR is safe to merge with no identified issues - The change is a pure security hardening that prevents misconfigurations. The validation logic is straightforward, follows existing patterns in the codebase, and has comprehensive test coverage. No breaking changes to existing valid configurations. - No files require special attention _{Last reviewed commit: a2003b4} <!-- greptile_other_comments_section --> <!-- /greptile_comment -->