feat(install): Add install script and GH page

[jdenquin]

Shell installer, served from GitHub Pages

Adds a curl | sh install channel so users get the prebuilt binary without a Go toolchain:

  curl -fsSL https://getlago.github.io/lago-cli/install.sh | sh

• install.sh (un-parked from dist-channels/parked/): picks the darwin/linux amd64/arm64 release tarball, verifies its SHA-256 against checksums.txt and the
  cosign signature when cosign is installed, installs to /usr/local/bin (sudo only if needed). LAGO_INSTALL_DIR and LAGO_INSTALL_VERSION override the defaults.
  Resolves latest via the releases redirect rather than the rate-limited API.
• pages.yml deploys the script to GitHub Pages on push to main, after shellchecking it and installing the latest release with it.
• release.yml gains a smoke-install-script job that installs from the Pages URL on ubuntu and macOS and gates the release.
• lago upgrade detects a script install and prints the curl line; unknown installs print all three commands. No self-update.
• test/docs guardrails updated: README must document three channels, and one test pins the URL across README, script, workflows and upgrade code.
• CI lints install.sh; README, CHANGELOG, DECISIONS updated.

Pages is already enabled on the repo with the Actions source (deploys allowed from main only). The URL goes live on the first pages.yml run after merge.

Tested locally: shellcheck clean, make lint and make test pass, and the script installed v1.0.0 end to end with a passing cosign check.