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).

skills/INDEX.md

@@ -280,6 +280,8 @@ Cross-cutting skills that apply to all pipelines:
 | Reviewer | `meta/reviewer.md` | Self-review protocol after every stage |
 | Checkpoint Protocol | `meta/checkpoint-protocol.md` | When/how to checkpoint and request human approval |
 | Skill Creator | `meta/skill-creator.md` | Dynamically create new skills during pipeline runs |
+| Animation Runtime Selector | `meta/animation-runtime-selector.md` | Choose render runtime + animation library per scene |
+| Bespoke Composition (Atelier) | `meta/bespoke-composition.md` | Hand-author a composition from scratch (hero work) — no stock scene-types; routes art-direction → motion principles → engine mechanics → atelier render |
 
 ## Style Playbooks