v0.7.1

chore(release): bump to 0.7.1 for corrected upstream release

Bumps Cargo.toml from 0.7.0 to 0.7.1 in preparation for tagging
v0.7.1. The v0.7.0 git tag was cut without bumping Cargo.toml
(`version` field stayed at 0.6.1), so the v0.7.0 release
tarball ships a binary whose --version reports 'lain 0.6.1'.

Cutting v0.7.1 with a properly-bumped Cargo.toml lets the
release workflow publish binaries that report the correct
version string. `server.json` gets updated to 0.7.1
automatically by the release workflow; `Formula/lain.rb` and
`npm-shim/package.json` remain at 0.6.1 and are out of scope.

Updates AGENTS.md to document the v0.7.0 → v0.7.1 progression.