Add Pre install adoption workflow

[charliechenye]

Summary

Adds the next adoption-focused improvements for SkillGate:

• Standardizes the locked uv development workflow.
• Adds skillgate review preinstall SOURCE.
• Adds stable redacted Markdown and JSON review packets.
• Automatically validates discovered Agent Skills for local inputs.
• Supports local files/directories, GitHub URLs, and local .mcpb bundles.
• Adds deterministic Markdown benchmark reporting.
• Adds a copyable pre-install starter repository.
• Clarifies local only behavior and optional GitHub integrations.
• Adds end-to-end adoption and workflow coverage.

Local-first behavior

Local scans:

• read local files only;
• write reports to the terminal or local paths;
• do not contact GitHub;
• do not upload SARIF or other artifacts;
• do not execute scanned content, install packages, or start servers.

GitHub URL scans and GitHub Actions integrations remain explicit opt-ins.

Compatibility

Existing commands and enforcement paths remain unchanged:

• scan
• check
• diff
• review summary
• github scan
• mcpb scan
• skills validate

Policy and baseline enforcement remains separate from advisory pre-install review.

Verification

• 386 pytest tests passed.
• Ruff lint and formatting passed.
• Golden snapshots passed.
• All 4 Node wrapper tests passed.
• Source distribution and wheel built successfully.
• Fresh environment CLI smoke test passed.

Scope exclusions

• No new rule families or rule IDs.
• No runtime execution or sandboxing.
• No telemetry.
• No PyPI or npm publication.
• No changes to the v0.1.2 tag.
• Package version remains 0.1.2; release versioning will be handled separately.