docs: fix scoped npm package references

[benjaminmodayil]

What

Update the remaining docs that reference the old/unscoped npm package name.

Changes in this PR:

• update the README npm badge/link to @open-gitagent/gitagent
• update docs.md installation command to npm install -g @open-gitagent/gitagent
• update examples/gitagent-helper/skills/get-started/SKILL.md installation command to npm install -g @open-gitagent/gitagent

Why

The repo still had a few references to the old package name / old npm link, while the published package is @open-gitagent/gitagent.

Closes #

How Tested

• npm run build passes
• gitagent validate passes on example agents
• Manual testing (describe below)

Manual testing:

• node dist/index.js validate -d ./examples/standard
• node dist/index.js --help

Checklist

• My code follows the existing style of this project
• I have added/updated tests (if applicable)
• I have updated documentation (if applicable)
• I have read the CONTRIBUTING.md