feat: ship skills/elastic/SKILL.md for agent invocation

`AGENTS.md` is for contributors and `cli-schema` is for introspection. Nothing in-repo tells an agent host how to *run* `elastic`. Contributor skills under `.claude/skills/` cover build and test only.

Add `skills/elastic/SKILL.md` (agentskills.io format) covering:

- Interactivity: never prompt; pass `--yes`; `--dry-run` before mutations
- Prefer `--json`; discover inputs with `--help --json` or `elastic cli-schema`
- Target a context with `--use-context`; do not pass credentials as flags
- Config / env: `ELASTIC_CLI_CONFIG_FILE`, `ELASTIC_CLI_TELEMETRY`, `ELASTIC_NO_BANNER`, `NO_COLOR`
- Diagnose with `elastic status --json` before retrying a failed API call
- Destructive confirm: `confirmation_required` means add `--yes`
- Do not dump `docs/cli/schema.json` into context; query the command you need

No `elastic skill install` command. Markdown in-repo plus a README pointer is enough for MVP.

Acceptance: one `SKILL.md`, linked from the README. An agent can configure a context and run a read command from the skill alone.