fix(release): make package-channel docs truthful

[Hmbown]

Closes #944.

Supersedes #946, which was closed when the branch was moved out of the v0.8.16 namespace for the hotfix lane.

Summary

• Document Scoop as a real but independently-updated Windows package channel that can lag GitHub/npm/Cargo releases.
• Stop presenting GHCR Docker images as an official supported release channel until publishing is working and public.
• Make the Docker release job non-blocking while Docker/GHCR remains experimental.

Live evidence

• npm: npm view deepseek-tui version --json returned 0.8.16 on May 7, 2026.
• Scoop main bucket: bucket/deepseek-tui.json returned 0.8.16 on May 7, 2026.
• GHCR: docker manifest inspect ghcr.io/hmbown/deepseek-tui:latest returned denied on May 7, 2026.

Test plan

• git diff --check
• ruby -e 'require "yaml"; YAML.load_file(".github/workflows/release.yml"); puts "release.yml ok"'
• cargo test -p deepseek-tui-cli update::tests::test_asset_matching_accepts_binary_assets_and_rejects_checksums --locked
• cargo fmt --all -- --check

[gemini-code-assist]

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