fix: Docker build shipping dummy binary, add release provenance

- Fix Dockerfile dependency caching: invalidate cargo fingerprints
  after copying real source so the actual binary is compiled
- Remove unnecessary libssl-dev (project uses rustls)
- Copy README.md into build context for include_str! macro
- Add generate-extras job to release workflow for shell completions
  and man pages (bash, zsh, fish, powershell)
- Add SLSA provenance attestation via attest-build-provenance for
  all release binaries, completions, and man pages
- Add social preview images and README infographic
- Bump version to 0.3.1

Author: zircote