chore(release): publish to crates.io via cargo-release

[bryantbiggs]

Summary

• Flips [package.metadata.release].publish from false to true in eksup/Cargo.toml so cargo release runs cargo publish as part of the release flow.
• Binary distribution (GitHub releases + Homebrew) continues to be driven by the existing tag-triggered release workflow — no CI changes needed.
• cargo-release's default ordering (bump → publish → tag → push) means a failed cargo publish aborts the release before any tag is pushed, so binaries + Homebrew can't drift ahead of crates.io.
• Verified locally with cargo publish -p eksup --dry-run --locked --allow-dirty (packages and compiles cleanly from the packaged tarball).

Closes #163.

Notes

• First publish will jump from 0.4.1 (last published 2023-11-28) to the next released version — non-sequential but legal on crates.io.
• No new secrets required: cargo publish uses local ~/.cargo/credentials.toml.
• The README's hardcoded version (README.md:33) is already kept in sync via the existing pre-release-replacements rule.

Test plan

• On next release, confirm cargo release publishes successfully to crates.io.
• After release, confirm cargo install eksup fetches the new version.