feat(compose): add atelier (bespoke) composition mode + doctrine

Introduce a hand-authored, project-local Remotion render path that bypasses
the cut-schema and the stock scene-type registry, for hero/bespoke videos
that must look distinct from one another.

- video_compose: composition_mode="atelier" (or renderer_family="bespoke")
  routes to _render_via_atelier, which renders a project-local entry under
  remotion-composer/projects/<slug>/ with an optional per-project public_dir
  (skips copying the bloated shared public/). No cut-schema, no stock registry.
- skills/meta/bespoke-composition.md: routing skill — art direction
  (visual-style) -> motion principles (Disney 12) -> engine mechanics
  (remotion-best-practices + stock components read only as a mechanics codex)
  -> atelier render. Doctrine: reuse engine knowledge, never creative components.
- AGENT_GUIDE: "Composition Authoring Mode" (templated vs atelier); default
  atelier for hero work; scene-type catalog reframed as a mechanics codex.
- animation-runtime-selector + INDEX: authoring-mode-first pointers.
- base_tool.run_command: decode subprocess output as UTF-8/replace (Windows
  cp1252 crashed the reader thread on Remotion's Unicode progress output).
- .gitignore: remotion-composer/projects/ (throwaway bespoke compositions).

.gitignore

@@ -81,6 +81,9 @@ node_modules/
 
 # Remotion build output and copied project assets (regenerable at render time)
 remotion-composer/out/
+# Project-local bespoke ("atelier") compositions — hand-authored per video,
+# throwaway and regenerable. Keeps the shared src/ free of one-off scenes.
+remotion-composer/projects/
 remotion-composer/public/*
 # But keep demo props (shipped with the project for zero-key demos)
 !remotion-composer/public/demo-props/