docs: install from GitHub instead of npm

[code-yeongyu]

Summary

• Install via pi install git:github.com/code-yeongyu/pi-rules (and the matching senpi install line). Remove the npm badge.
• Keep manual install as a git clone plus npm install inside the clone.
• Skip npm publish when NODE_AUTH_TOKEN is unset so release runs exit 0 instead of failing with ENEEDAUTH.
• Record the docs and CI changes under Unreleased.

Refs #32

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Summary by cubic

Switches the documented install path from npm to GitHub and lets the release workflow succeed without publishing when no npm token is configured.

• README now points to pi install git:github.com/code-yeongyu/pi-rules, adds the matching senpi install line, and drops the npm badge.
• Manual placement stays a git clone plus npm install inside the clone.
• The publish job now exits 0 instead of failing with ENEEDAUTH when NODE_AUTH_TOKEN is unset.
• Records both changes under Unreleased in the changelog.

^{Written for commit 91037c4. Summary will update on new commits.}