docs: bring README, CLAUDE.md, and CHANGELOG up to date Verified

130 commits past v0.3.0 had landed without any doc updates. README
features list, architecture file list, and production-deployment
instructions were all out of date — cavectl, Pulse, releases,
provenance, the public landing, the clone widget, ghcr.io images,
external SMTP, and a half-dozen Pulse/release/cavectl additions
were nowhere in the docs.

- README.md: rewrite features list, add cavectl path A and quadlet
  path B for deployment, document published container images, fix
  the architecture file list (chamber, workflow, runner-service,
  metrics, search-zoekt etc. were missing), show both cave.conf and
  cave.yaml as the two configs that live side by side, and add
  cavectl subcommands to the CLI section.
- CLAUDE.md: drop the "cave init --admin-user" snippet (cave uses
  OIDC, no local admin), expand the file list, add conventions for
  chamber-or graceful fallback, trailing-slash redirects, the
  cave-shell two-line stdout protocol, and the ghcr.io image
  publication path.
- CHANGELOG.md: draft v0.4.0 (unreleased) from the git log grouped
  by feature, plus a v0.3.0 summary and a placeholder for v0.2.0.
  The v0.1.0 *Poof* entry stays for posterity.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>