Move PHP/FrankenPHP binaries to a dedicated 'artifacts' release

[munezaclovis]

Summary

• Decouples FrankenPHP + static PHP CLI binaries from pv's versioned releases by moving them to a single non-versioned artifacts release (marked --latest=false so v* stays the user-facing latest).
• Simplifies build-frankenphp.yml: creates the artifacts release if missing, then clobbers on every rebuild — no more hunting for the "latest v* tag".
• Simplifies release.yml: drops the forward-copy step entirely. pv releases now ship only the pv binary.
• Simplifies the Go install path: install.go and available.go hit releases/download/artifacts/... directly; latestReleaseTag() is gone, along with its encoding/json/io imports.

Why

FrankenPHP and PHP CLI binaries have their own lifecycle (weekly cron against php/frankenphp:latest). Previously they were attached to whatever pv v* release happened to be "latest", and every new pv tag had to babysit a forward-copy step just to preserve install URLs. That meant:

• pv release notes polluted with unrelated binary assets
• install URLs pointing at version tags that have nothing to do with binary versioning
• a copy step in release.yml that's silently load-bearing for installs

Separation fixes all three.

⚠️ Transition (clean break)

The artifacts release does not exist yet. Before merging / tagging the next pv version:

1. Seed the artifacts release. Either:
   • Trigger build-frankenphp.yml on main (manual workflow_dispatch) after merge. The workflow now creates the release on first run. Until it completes, pv php:install on the new code will 404.
   • Or manually: gh release create artifacts --latest=false --title \"PHP binaries\" --notes \"...\" and copy current php-* / frankenphp-* assets from v0.0.5 into it.
2. Then tag v0.0.6 with the new install code.

Pre-v0.0.6 pv installs already out in the wild are unaffected — they download from their original v* URLs. The clean break only matters for new installs on v0.0.6+.

Files

• `.github/workflows/build-frankenphp.yml` — uploads to `artifacts` tag
• `.github/workflows/release.yml` — copy step removed
• `internal/phpenv/install.go` — uses `artifactsTag` constant; `latestReleaseTag()` removed
• `internal/phpenv/available.go` — queries `/releases/tags/artifacts` directly

Test plan

• `gofmt`, `go vet`, `go build`, `go test ./...` clean locally
• Both workflow YAMLs parse
• After merge, `workflow_dispatch` of build-frankenphp.yml on main creates the `artifacts` release and uploads all 6 assets
• `gh release view artifacts` shows `isLatest: false` and expected asset list
• Tagging v0.0.6 publishes only the pv binary (no frankenphp/php-* assets on the v* release)
• `pv php:install 8.4` on a fresh install using v0.0.6 successfully downloads from the `artifacts` release