Add the ellipsis Agent Skill: teach any coding agent the platform + CLI

[hbrooks]

What

A public Agent Skill at skills/ellipsis/SKILL.md that teaches any coding agent (Claude Code, Cursor, Codex, and others) what the Ellipsis platform is, when to reach for it, and how to drive it with the agent CLI. Plus a README section telling users how to install it.

Distribution: alternatives considered

1. Open Agent Skills layout + npx skills add (chosen). The Vercel-labs skills CLI installs skills/<name>/SKILL.md from any GitHub repo into 70+ agents (Claude Code, Cursor, Codex, Cline, ...), and skills.sh auto-lists installed repos. One layout also serves manual copy into ~/.claude/skills/ and the dogfood path: an Ellipsis agent config can load the skill from this public repo with a skills: entry, no clone needed.
2. Claude Code plugin marketplace (.claude-plugin/marketplace.json + /plugin marketplace add): Claude-Code-only and heavier than one skill warrants; can be layered on later without moving files.
3. Bare repo + manual copy only: works but gives up the one-command install and the skills.sh listing.

Hosting it in this repo (rather than a new ellipsis-dev/skills repo) keeps it next to the CLI it teaches, in the public repo users already install from.

Verification

• Every CLI verb quoted is verified against this repo's source at HEAD (session start/list/get/search/steps/connect/stop/replay/ide/port/handoff, hooks install, config init/create, template list, sandbox variable set, asset upload, analytics).
• The YAML example parses as a deployable config with the monorepo's real validator (parse_agent_config_yaml).
• npx skills add . -l on this branch discovers the skill with the right name/description.
• Docs URLs referenced all return 200; copy follows the marketing rules (canonical positioning verbatim, no em-dashes, no dollar-figure or gated-feature claims).

CI is unaffected (markdown only).