#16362: Update session-pruning.md - updated agents.defaults config std

## Summary Updated documentation to match config standard. agent.* was moved to agents.defaults. - Problem: - Why it matters: docs were outdated - What changed: examples to match current config format - What did NOT change (scope boundary): descriptions and text outside of the code examples ## Change Type (select all) - [ ] Bug fix - [ ] Feature - [ ] Refactor - [x] 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 none, docs only ## Security Impact (required) none ## Repro + Verification N/A ### Environment N/A ### Steps N/A ### Expected N/A ### Actual N/A ## Evidence N/A. Docs ## Human Verification (required) Docs ## Compatibility / Migration Docs ## Failure Recovery (if this breaks) N/A Docs ## Risks and Mitigations None <!-- greptile_comment --> <h3>Greptile Summary</h3> Documentation-only change that updates the three json5 code examples in `docs/concepts/session-pruning.md` from the deprecated `agent:` config key to the current `agents.defaults` structure. The updated examples now match the canonical configuration reference at `docs/gateway/configuration-reference.md` and the source code in `src/config/defaults.ts`. No text or descriptions were changed outside the code blocks. - Updated `agent:` → `agents: { defaults: { ... } }` in all three code examples (off mode, cache-ttl mode, and tool-scoped pruning) - Note: the zh-CN translation (`docs/zh-CN/concepts/session-pruning.md`) still uses the old format but is auto-generated and will be updated when the i18n pipeline runs <h3>Confidence Score: 5/5</h3> - This PR is safe to merge — it only updates documentation code examples to match the current config format. - Documentation-only change with no code impact. The updated examples are verified against both the configuration reference doc and source code. The changes are minimal, correct, and well-scoped. - No files require special attention. _{Last reviewed commit: b2b6f95} <!-- greptile_other_comments_section --> _{(2/5) Greptile learns from your feedback when you react with thumbs up/down!} <!-- /greptile_comment -->