docs: add npm install flow for 0.5.26

[ByBrawe]

Prepare OpenCode Loop 0.5.26 as a documentation/install patch release.

• Add a clear global npm installation path alongside the recommended npx installer.
• Document npm update and uninstall flows.
• Clarify that project-local npm install alone does not run the OpenCode installer.
• Add matching npm installation instructions for the recommended dedicated OpenCode Goals plugin.
• Update release notes for 0.5.25 and 0.5.26.

Merge after PR CI passes, then publish 0.5.26 from the release branch against the updated main.

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