feat: add Neovim what's-new learning skill

[zimakki]

Summary

• add the personal zimakki-nvim-update-brief skill for learning newly possible Neovim workflows without updating anything
• collect Lazy and Mason facts with a dependency-free Elixir helper and per-component coverage memory
• curate at most seven strong discoveries and require a dedicated zimakki-html-doc presentation subagent
• reject v4 targets and verify lockfile, plugin, and Mason state before advancing coverage
• preserve the earlier Python feature branch and worktree as reference

Verification

• elixir tests/skills/zimakki_nvim_update_brief_test.exs — 16 tests passed
• scripts/ci_checks.sh — passed
• Elixir script plus tests — 988 lines, below the 1,000-line design cap
• live astronvim_v6 collection — 60 Lazy plugins and 9 Mason tools; guard, config status, and lockfile unchanged
• blind forward test — selected six workflow discoveries, classified maintenance separately, left unresolved components pending, and withheld completion until HTML verification

Notes

The skill is report-only. It never invokes Neovim, Lazy, Mason, package installers, or mutating Git commands.