CRAN prep 0.7.1: title, acronym expansion, ?saber page

[TroyHernandez]

Summary

• Title: 'Context Engineering for R' → 'Context Engineering for Large Language Model Agents'. Drops the 'for R' trailing redundancy that CRAN cookbook flags (corteza 0.6.x precedent).
• DESCRIPTION: expand 'AI' and 'AST' on first use per CRAN spell-check policy.
• Package help: add R/saber-package.R so ?saber returns a help page.
• NEWS.md and cran-comments.md updated.

Pre-submission review punch list (full): Title, AST, AI, _PACKAGE doc. Also evaluated a set.seed save/restore in graph_svg() but reverted — the restore pattern requires explicit .GlobalEnv writes that trip CRAN lint, and the cookbook explicitly allows set.seed() inside a function that takes a user seed arg.

Test plan

• tinypkgr::check() — 0 errors, 0 warnings, 0 notes
• tinypkgr::check_win_devel() (next, after merge)