Make slice add output concise

[NagisaVon]

Summary

• render slice add as a concise mutation acknowledgement instead of repeating the full atom inventory
• preserve the frozen add_slice(...) -> StatusResult facade and all v0 JSON/model/store contracts
• add a 163-atom CLI regression covering the large-change failure mode found in the PR #2404 experiment

Design review

• implements the approved A18 delta-oriented mutation rendering decision
• remains consistent with Product Design 5.25/5.26 and Implementation Plan 3.1/A18
• intentionally leaves README and SKILL.md unchanged; T13 owns public documentation after T12 CLI text is complete
• this PR is the first independently mergeable T12 increment, not completion of the full selectors/batch/CRUD workstream

Validation

• pytest -q — 104 passed
• ruff check src tests
• ruff format --check src tests
• mypy src tests
• git diff --check origin/main...HEAD