feat: add install command for single package install

[maiconfz]

Workflow

Branch pattern: feat/<issue-number>-install-single-package (use the assigned number and a short lowercase kebab-case slug)

Draft PR: Open with gh pr create --draft before implementation commits. Use Closes #<issue-number> in the PR ## Related Issues section.

Summary

Add npx agents-repo install <package> / i alias for single package install.

Motivation

Core MVP command to install registry packages into projects (skills.sh / npm install parity).

Proposed scope

In scope:

• Resolve registry source from ConfigResolver + webapp-style ref/tag resolution
• Resolve package ref (qualified id / alias) from index
• Resolve version via manifest + semver range
• Build artifact URL with buildRegistryArtifactUrl
• Validate compatibility.targets / index installTargets includes chosen target
• Download ZIP, verify sha256, ZIP security scan (registry zip-scan rules), extract per install-targets
• Update packages + agents-lock.json unless --no-save
• Flags: --agent, --flow, -g/--global, -y, --target, --dry-run, --no-save
• Reject yanked; warn deprecated

Out of scope:

• Bulk install (no args)

Impacted areas

• src/modules/install/
• src/modules/cli/presentation/

Done criteria

• Integration test with mocked registry HTTP
• Installs known package into temp dir
• Rejects malicious ZIP fixture

Milestone

M3 — MVP CLI

Depends on

Registry client issue, config/lock management issue