docs: document YAML as the default desired-state language

[amiorin]

The CLI has defaulted to green.yml since read-state started picking the
reader from the file extension, but the README and the specification still
described desired state as EDN and printed green.edn in the usage line.

• README: "desired state in YAML or EDN", plus a bullet covering the
  default file, the extension-based reader, and the COLORS_PAR_* channel.
• index.html: correct the philosophy bullet and the section 2 lead, add a
  reader table, and fix the usage synopsis and examples in section 10.
• index.html: document the COLORS_PAR_* secret channel, which arrived with
  the YAML reader and was never written up, and add :green/state-file to
  the reserved-key table.
• index.html: add read-state, read-pars and par-name to the API index, list
  the four namespaces missing from the layout tree, note yamlstar in the
  deps, and cover cli/yaml/process in the testing section.
• index.html: say plainly that the in-repo examples stay on green.edn while
  downstream packages use colors.yml, so both halves stay exercised.

EDN is still read for any non-.yml/.yaml extension, so the docs describe a
default rather than a replacement.

Co-Authored-By: Claude Opus 5 noreply@anthropic.com
Claude-Session: https://claude.ai/code/session_01CJ3gwMLE1pmNLDPgbtECYg