feat: add Breakdown connectivity skill

[grid-breakdown]

Summary

• Add a root-level breakdown-connectivity Agent Skill for evidence-first macOS, network, API, and MCP diagnosis.
• Document canonical Block collection and upstream Breakdown install paths, including the Claude Code plugin path.
• Add a focused MCP tool reference covering current health, historical issues, targeted evidence, analysis, and Evidence Reports.

Structure Checklist

• Skill is in a top-level folder: breakdown-connectivity/
• Folder contains SKILL.md
• SKILL.md includes name, description, author, version, and tags
• Supporting reference is inside the skill folder

Security Acknowledgement

• No malicious behavior
• No secret, token, credential, or private-data exfiltration
• No obfuscated payloads or binaries
• I understand automated validation and security scanning will run
• I understand this PR may be rejected if it fails validation or security review

Test Plan

• node scripts/validate-skills.mjs --skills breakdown-connectivity — passed.
• Canonical source tests from PeaceCraft-LLC/breakdown-agent-connectivity — tests/setup-breakdown-test.sh and tests/distribution-test.sh passed.
• scripts/skills-security-scan.mjs report for this skill: WARN, FAIL=0, WARN=3, FILES=2; the current script then exits with a pre-existing ReferenceError: sks is not defined at line 851 after writing the report.

The three warnings are expected URL-review findings for the documented install/download/agent-guide URLs. No remote-execution or secret-target findings were reported.

[grid-breakdown]

Contributor verification update:

• DCO now passes after adding the Signed-off-by trailer.
• Local npm run validate completed successfully for all 8 skill directories.
• The remaining validate and security_scan failures occur before repository code runs: actions/checkout@v4 refuses fork PR code inside the repository’s pull_request_target workflows (allow-unsafe-pr-checkout: false), so no report is produced. This appears to be a workflow configuration issue rather than a finding in this contribution.

I have not modified workflow files in this PR.