ci: keep validation fast and scope compatibility checks

[SanjithSambath]

Summary

• keep validate as the single fast, deterministic check on every pull request
• run SDK, toolkit, webhook, and CLI compatibility checks only when their pins, fixtures, or workflow change
• run marketplace installation checks only when plugin/skill packaging changes, weekly, or manually
• keep weekly package-version drift detection, and also validate pin/checker changes on pull requests
• remove MCP/spec probes from this repo because agentmail-mcp owns those public surfaces

Why

The previous workflow installed several external CLIs and package graphs on every change. These checks remain intact, but now run where they can catch relevant regressions. Fast repository validation stays always-on.

Validation

• python3 scripts/validate_repo.py
• python3 scripts/check_compatibility.py
• parsed every workflow as YAML
• verified workflow pin extraction from compatibility.json

Draft only; do not merge automatically.