Purge facets references from docs

[jgs-jeeves]

Summary

Facets are deprecated in favor of --context, but the stan-cli documentation still presents facets as an active feature. This creates confusion and misdirects users toward a workflow that is no longer intended.

References to remove / rewrite

At minimum:

• guides/stan-assistant-guide.md

  • “Facet overlay” mental model section
  • flags: -f, --facets, -F, --no-facets, --facets-on, --facets-off
  • facet files: facet.meta.json, facet.state.json, .docs.meta.json overlay metadata

• guides/cli-examples.md

  • “Facet overlay (selection view…)” section
  • example commands using --facets-*

• guides/archives-and-snapshots.md

  • “Overlay-aware snapshots” section (if overlay/facets are fully deprecated)

(There may be additional mentions in README or other guides; do a repo-wide search for facet, facets, facet.meta.json, and --facets.)

Desired outcome

• Docs should not recommend or describe facets as a current feature.
• Replace guidance with the canonical --context approach:
  • when to use --context
  • --meta archive concept (thread opener / minimal context)
  • how to reduce context size / improve relevance with context mode rather than overlay exclusions

Acceptance criteria

• A search for facet / facets / --facets in guides/ returns no results, or only clearly-labeled deprecation notes that point to --context.
• CLI examples no longer reference facet flags.
• The assistant guide’s “mental model” aligns with current implementation strategy (--context over overlay).

Notes

This issue is intentionally scoped to documentation cleanup; a broader “docs aligned to implementation” sweep can follow as a separate issue/epic.