v3.0.0: AgentFeedback: new home, new names, fresh start

The project moved to github.com/AgentFeedback/agentfeedback
and is the open-source, self-hostable AgentFeedback
(agentfeedback.dev); a hosted version with the
same API runs at agentfeedback.io. Every name
changes and v3 is a fresh start: no migration from or compatibility with
2.x deployments and skill installs. The HTTP API (1.1), payloads and hash forms are unchanged.

• Skills agent-feedback → agentfeedback 4.0 and
  agent-feedback-triage → agentfeedback-triage 3.0: new directory and
  invocation names (/agentfeedback-triage). Client environment variables
  keep their names (AGENT_FEEDBACK_*); the spool moves to ~/.cache/agentfeedback and triage digests to
  ${TMPDIR:-/tmp}/agentfeedback-triage/. The skills work against a
  self-hosted service or https://api.agentfeedback.io.
• Service: Go module github.com/agentfeedback/agentfeedback; binary
  agentfeedback (cmd/agentfeedback, /opt/agentfeedback, runs as user
  agentfeedback); image ghcr.io/agentfeedback/agentfeedback; default
  database /data/agentfeedback.db; metrics renamed agentfeedback_*
  (agentfeedback_submissions_unprocessed, agentfeedback_db_bytes,
  agentfeedback_sqlite_busy_total).
• Deploy: stack directory infra/agentfeedback, compose service
  agentfeedback, volume agentfeedback-data, server directory
  ~/agentfeedback, variables AGENTFEEDBACK_IMAGE and
  AGENTFEEDBACK_BIND_ADDRESS.
• Removed: the 1.x PostgreSQL export script and migration and uninstall
  docs, and the docs/agent-usage.md redirect.
• CI lowercases the image name (the org name has capitals).