v1.0.8 — document env-var auth for headless/agent runtimes

- SKILL.md: document `export CONTENTSTUDIO_API_KEY` as an auth path
  alongside `auth:login --api-key`; the env var takes precedence over the
  CLI config file.
- SKILL.md: add a headless-deployment note — a shell `export` does not
  persist to a service, so set the variable via systemd `Environment=` /
  `EnvironmentFile=`, Docker `-e`, etc. Runtimes that gate on
  `requires.env` (e.g. OpenClaw) stay blocked until it is present.
- Resolves a docs/metadata mismatch: the frontmatter already declared
  `requires.env: CONTENTSTUDIO_API_KEY`, but the body only documented
  `auth:login`, leaving headless operators blocked with no guidance.
- Bump version to 1.0.8 (SKILL.md frontmatter, package.json) and add
  CHANGELOG entry. No CLI source changes.