[feat] Telescope support for markdown and pipeline preview kinds

[adriancmiranda]

Context

The Telescope integration currently exposes toggles for image, video, archive, sqlite, font, cert, key, binary, markdown, and model previews. Glimpse also supports PlantUML and Mermaid, but these pipeline-based kinds are not configurable or rendered through Telescope today.

Goal

Extend the Telescope integration to support:

• markdown
• model
• plantuml
• mermaid

Completed

• Markdown Telescope previews were added in feat(telescope): render markdown previews #133.
• Markdown now falls back to raw file contents when no Markdown renderer is installed in fix(markdown): fall back to raw text without renderer #134.
• 3D model Telescope previews were added through the shared pipeline preview path in feat(telescope): render model previews #135.

Notes

Markdown uses the text preview path through preview_data.

Model previews use the shared pipeline previewer with an explicit Telescope preview buffer/window target.

PlantUML and Mermaid use pipeline-based previewers, so Telescope still needs shared async/render support for those kinds without duplicating pipeline logic or rendering stale results.

Tasks

• Add Telescope config toggle for markdown.
• Render markdown through the text preview path.
• Preserve fallback behavior for markdown when no renderer is installed.
• Update README and vimdoc for markdown.
• Add markdown regression tests.
• Add Telescope config toggle for model.
• Add pipeline preview support for model in Telescope.
• Preserve fallback behavior when model is disabled.
• Update README for model.
• Add model Telescope regression tests.
• Add Telescope config toggles for plantuml/mermaid.
• Add pipeline preview support for plantuml/mermaid in Telescope.
• Preserve fallback behavior when plantuml/mermaid are disabled or unavailable.
• Update README and vimdoc for plantuml/mermaid.
• Add PlantUML/Mermaid Telescope regression tests.