Trim duplicated vercel flags CLI docs from flags-sdk skill (#492)

* Trim duplicated vercel flags CLI docs from flags-sdk skill

Point agents at --help for syntax and keep lifecycle/safety guidance that skills must own (EXP-3411).

* Shrink flags-sdk skill description under 1024 chars

Drop duplicate CLI trigger phrases; keep subcommand names once for activation.

* Drop stale skill-tables wording from CLI pointer

* Align flags-sdk skill with EXP-3411 acceptance criteria

Make --help discovery and the vercel-cli cross-link explicit in SKILL.md and providers.md.

* Trim frontmatter subcommand list and drop duplicated CLI prerequisites

* Add CLI-to-SDK glue and existing-flag flow to flags-sdk skill

* Address review: restore rm/unarchive/open in frontmatter, fix split default-variant wording

* Correct set/use-targeting semantics: targeting is paused, not replaced

* Ground CLI lifecycle guidance in public Vercel Flags docs

* Align authentication guidance with vercel.com docs: OIDC default, SDK keys manual

* Fix singleton client auth note; drop nonexistent targeting subcommand

* Consolidate env pull guidance into one section