Add collaboration docs scaffold with module index and issue/todo split

[Copilot]

This PR introduces a shared, repo-local documentation baseline for LLM-assisted collaboration, with a consistent structure under docs/. It also adds a dedicated docs/sources/ area to document package modules and clarifies the role of issues.md as a curated working subset distinct from todo.md.

• Documentation foundation (docs/)

  • Added first-draft core documents:
    • agents.md (agent working guidelines)
    • context.md (project/package/test layout)
    • decisions.md (key process/design decisions)
    • ideas.md (speculative directions)
    • invariants.md (hard repository constraints)
    • issues.md (curated selected issues)
    • todo.md (concrete next actions)
    • references.md (external links)
    • primer.jj (Jinja collaboration primer)
    • index.md (top-level docs/source/test map)

• Source documentation expansion (docs/sources/)

  • Added docs/sources/index.md with a factual module-level inventory of:
    • top-level package modules (e.g. caching.py, metrics.py, sampling.py, schedules.py, etc.)
    • subpackage module files for blocks, layers, maths, models, and shaping

• Operating model for issue tracking docs

  • issues.md now defines a curated-priority list for active focus.
  • todo.md remains the execution list for concrete next tasks.
  • This keeps GitHub Issues as the full system of record while enabling a concise in-repo working set.

docs/
├── agents.md
├── context.md
├── decisions.md
├── ideas.md
├── index.md
├── invariants.md
├── issues.md
├── primer.jj
├── references.md
├── todo.md
└── sources/
    └── index.md

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