Agent instructions load progressively as skills

The contributor guide is no longer one always-on file. Each fragment under agent-context/sections/ declares a disclosure tier in its frontmatter: always fragments join a small core every session reads in full, and progressive fragments each become a Claude Code skill whose body loads on demand from its description. There are no committed AGENTS.md / CLAUDE.md files anymore; a SessionStart hook renders the core live and points .claude/skills at a generated link farm.

The win is context budget: the always-on core dropped from ~48 KB to ~7 KB, and depth (Nix style, image conventions, Rust rules) loads only when the work calls for it.

The size of the always-on tier is a build-time invariant. Marking too much always fails the build with a clear message instead of silently overflowing Claude Code’s per-value context cap.

nix build .#claude-md --no-link --print-out-paths | xargs cat   # always-on core
nix build .#skills    --no-link --print-out-paths | xargs ls    # handwritten + generated skills
nix run  .#agent-context -- --write                             # write the core to disk (gitignored)

Sibling repos consume index.lib.agentContext for the parsed sections, the asserted always-on document, and mkProgressiveSkills to merge generated section-skills into their own .claude/skills.