v2.2.1: Documentation sweep

Docs and skill documentation only; the service and scripts behave as in
v2.2.0.

• Skill agent-feedback-triage 2.1: the optional clustering contract
  moved to reference/clustering.md, read only when clustering is used, and
  repeated rules were collapsed: SKILL.md is a third shorter per invocation.
  Install the whole directory, including reference/. Resolutions now start
  with their verdict (FIXED, INVALID, DUPLICATE-OF-<id>). Fixed: the
  close-out commands' path to the sibling skill, digest exit codes, the
  meaning of unchanged versus updated, and cluster.py's statuses, reasons
  and exit codes.
• Skill agent-feedback (contract unchanged): corrected the machine
  override, --sweep age/lock/outcome behaviour, backlog-warning scope, TSV
  column 5, slots usage, 1xx/3xx outcomes, export digest tools, and added
  AGENT_FEEDBACK_TRIAGE_ROOTS.
• api.md states its version (1.1), the non-JSON responses and the
  include=payload row shape.
• operate.md: the retention purge runs as root, backups create
  their directory, openssl and DEPLOY_DIR are documented, and uninstall
  covers the triage variables.
• CLAUDE.md routes to develop.md, the single home for
  commands, rules and gates; the README quickstart checks out the latest tag
  and keeps an existing key.