docs(skills): collapse three overlapping skills trees into one verified tree

Three parallel trees documented the same ecosystem: omp-docs/skills/
(two overview pages), tools/npx-skills/ (10 pi-era npm pages), and
tools/vercel-labs-skills/ (11 CLI pages), plus a fourth leaderboard page
at engineering/skills-ecosystem.md. Verbatim overlap (identical SKILL.md
field tables in skills-spec.md vs skill-format.md; 13-agent vs 68-agent
copies of the same upstream table). Consolidated to four concept pages:

- skills/agent-skills.md: standard, format, loading, authoring (merges
  skills-spec, skill-format, skill-file-structure, skill-loading,
  best-practices, skills-vs-extensions)
- skills/skills-cli.md: CLI reference (merges commands, source-formats,
  installation-scope, skill-discovery, lock-file, env-variables,
  agent-support, related-links)
- skills/store-wiring.md: moved, fleet ops, links retargeted
- skills/ecosystem.md: replaces skills-ecosystem.md; stale 2026-08-12
  top-50 table dropped, replaced with a live top-10 re-queried from the
  skills.sh API tonight and the API contract re-verified (zero-based,
  200/page, {skills,total,hasMore,page}, total 9909)

All facts re-verified 2026-10-19 against primary sources:
agentskills.io, vercel-labs/skills README + src/cli.ts (command set,
--full-depth) + src/skill-lock.ts (v3 lock, XDG path, wipe-on-old),
live skills.sh API. Deleted superseded core/index-doc.md (old pi-era
docs index). Retargeted all inbound links (tools/index, engineering x3,
packages x3, pi/skills, core/index). Pi-era npm skill packaging is
listed as archived under /pi/.

Refs #29

omp-docs/skills/index.md

@@ -1,11 +1,14 @@
 # Skills Documentation
 
-Distribution and installation patterns for agent skills via npm packages, including SKILL.md structure, discovery rules, and the pi-package system.
+The open agent-skills ecosystem as the fleet uses it: the standard, the
+installer CLI, this fleet's provisioning tiers, and the registries.
 
 ## Concept Files
 
-| File | Type | Description |
-|------|------|-------------|
-| [npx-skills.md](npx-skills.md) | reference | Distribution and installation patterns for agent skills via npm packages, including SKILL.md structure and the pi-package system |
-| [vercel-skills.md](vercel-skills.md) | reference | The meta-installer CLI for cross-agent skill distribution, supporting 68+ agents |
+* [The Agent Skills Standard](agent-skills.md) — SKILL.md format, frontmatter fields, loading model, authoring practice, skills vs extensions.
+* [Skills CLI](skills-cli.md) — `npx skills`: commands, sources, scopes, discovery, lock file, agent registry.
+* [Skill Store Wiring](store-wiring.md) — the three skill tiers on Dank workstations and how omp loads them.
+* [Skills Ecosystem](ecosystem.md) — registries, the skills.sh leaderboard API, live install snapshot.
 
+Pi-era skill packaging (`pi install npm:...`, the `pi` package manifest) is
+archived under [pi](/pi/index.md).