ci: add release flow modeled on git-workflow

[zawakin]

Adds a release flow in the same style as the git-workflow repo, adapted to rep.

What's included

• /release skill (.claude/skills/release/SKILL.md) — bump Cargo.toml, run mise run rep:verify, commit chore: release vX.X.X, tag, and create the GitHub release.
• .github/workflows/release.yml — on a v* tag: verify the tag matches the Cargo.toml version, build binaries for 5 targets (macOS Intel/ARM, Linux x64/ARM64, Windows x64), and attach them + install.sh to the GitHub release (softprops/action-gh-release).
• .github/workflows/ci.yml — test (3 OSes) / fmt / clippy / build / lockfile on push & PR to main.
• install.sh — platform-detecting installer for the prebuilt binaries.
• Docs: README install section, CLAUDE.md release section + write-exception note, tasks.md pointer.

Differences from git-workflow's flow

• Single binary (rep) instead of two — packaging and install.sh adjusted.
• No crates.io publish (per decision) — no CARGO_REGISTRY_TOKEN needed; the GitHub-release job depends only on the build job, so releases never hinge on a publish step.
• Tag-vs-version check kept as a dedicated verify-version job that gates the build.

Notes for the maintainer

• The release skill creates the GitHub release with curated notes; the workflow then attaches binaries to that same tag's release (idempotent).
• mise run rep:verify is green locally (38 tests). The workflows themselves run on GitHub once merged.

🤖 Generated with Claude Code