ci: add CI, release, and tag-release workflows + GitHub Pages landing page

[relentlessworks]

What's in this PR

CI Workflows

• ci.yaml — Lint (go vet, govulncheck), test (race mode), build, and cross-compile for linux/amd64, linux/arm64, darwin/amd64, darwin/arm64, windows/amd64
• release.yaml — Triggered on v* tags. Builds binaries for all platforms with -ldflags "-s -w", packages as tar.gz, creates checksums, and publishes a GitHub Release
• tag-release.yaml — Auto-tags on merge to main by reading the VERSION file, then dispatches the release workflow

GitHub Pages Landing Page

• docs/index.html — A polished, dark-themed static landing page for LinkSmith with hero section, quick start, principles grid, API reference, configuration, and build instructions

VERSION file

• Added VERSION file (0.1.0) used by the tag-release workflow