Fix collector release: tag derivation + decouple GitHub release

[galacto-last9]

Why

collector-release.yml failed. Two distinct problems surfaced across the failed runs, plus a latent third:

1. GHCR push denied (the active blocker): denied: permission_denied: write_package pushing ghcr.io/last9/l9gpu-collector. The package doesn't exist yet and GITHUB_TOKEN is refused creation rights under the last9 org. This is an org-settings fix, not code — see below.
2. Tag derivation broke on workflow_dispatch: github.ref_name is main, so GORELEASER_CURRENT_TAG became main, producing a :main-amd64 image.
3. Release-tag collision (latent): goreleaser (OSS) derives the GitHub release tag from the parsed semver (v0.2.0), which is already owned by the Python release.yml. Both collector-v0.2.0 and v0.2.0 point at the same commit, so collector artifacts would be attached to the Python release.

Changes

• Tag derivation: use the tag on a tag push; fall back to the latest collector-v* tag on workflow_dispatch/branch runs (fail loudly if none).
• Decouple the GitHub release: set release.disable in .goreleaser.yml and publish archives + checksums via gh release create at the namespaced collector-vX.Y.Z tag (idempotent on re-runs). goreleaser still builds binaries and pushes the docker image. (monorepo.tag_prefix, the clean fix, is goreleaser Pro-only and rejected by the OSS build CI uses.)
• Docs (docs/COLLECTOR.md): correct the pinned docker tag format (no collector- prefix on the image tag) and the multi-arch claim (image is amd64-only; arm64 ships as a binary tarball).

Still required (out of band, org admin)

The GHCR permission_denied is not fixed by this PR. An org owner must allow Actions to create/write the l9gpu-collector package (Org → Settings → Actions → workflow permissions = read/write, and package creation enabled), or pre-create the package and grant last9/gpu-telemetry Write under the package's Actions access.

Validation

• .goreleaser.yml validated with goreleaser check (OSS, ~> v2) — passes (release pipe skipped as intended).
• Workflow YAML parses.