chore(release): prefix --version + release artifacts with v; cut v0.2.1

[proofmancer]

Summary

Adopts a v prefix consistently across the public version surface so binary, release tag, and release-asset filenames all agree. No behaviour or wire-format changes.

Cuts v0.2.1.

Why

The release tag has always been vX.Y.Z, the binary's --version printed ligate X.Y.Z, and asset filenames were ligate-cli-X.Y.Z-PLATFORM.tar.gz. Three different spellings of the same version was confusing; the README install snippet also drifted from the actual asset filename pattern (missing the cli- infix), so the documented curl URL 404'd.

Diff

File	Change
.github/workflows/release.yml	Drop the VERSION="${VERSION#v}" strip in "Compute version". Tag is used verbatim; assets become ligate-cli-vX.Y.Z-PLATFORM.tar.gz.
src/cli.rs	Replace version, with version = concat!("v", env!("CARGO_PKG_VERSION")). ligate --version now prints ligate vX.Y.Z.
README.md	Install snippet: filename pattern fixed (ligate- → ligate-cli-), platform list trimmed to the three actually built (no darwin-amd64), example + "Current release" bumped to v0.2.1.
Cargo.toml + Cargo.lock	0.2.0 → 0.2.1.
CHANGELOG.md	New [0.2.1] entry.

Test plan

• cargo fmt --check clean (pre-commit gated too)
• CI green
• After merge + tag v0.2.1: release workflow produces ligate-cli-v0.2.1-linux-amd64.tar.gz (and arm64 + darwin-arm64 siblings, plus .sha256 sidecars)
• Installed binary reports ligate v0.2.1 from --version
• README curl pattern (.../v0.2.1/ligate-cli-v0.2.1-linux-amd64.tar.gz) resolves to a real asset

Forward applicability

Effective from v0.2.1 onward. v0.2.0 and earlier tarballs keep their existing names; no retroactive rename.

Cross-repo

ligate-marketing apps/landing/src/app/devnet/page.tsx:199 still shows ligate-cli 0.1.3 (no v prefix). Followup in that repo, separate PR.