docs: update post-release status

[CoderDeltaLAN]

Summary:

• Update README status after publishing v0.1.0 as a GitHub pre-release.
• Add basic installation guidance for GitHub Release artifacts.
• Record the post-release README update in CHANGELOG.md.

Scope:

• Documentation only.
• No code changes.
• No CI changes.
• No release/tag changes.

Validation:

• ./scripts/check.sh passed.
• git diff --check passed.
• Pre-push checks passed.

Risk:

• Low; documentation wording only.

Rollback:

• Revert this PR if the README should return to release-candidate wording.