fix(installer): dump gateway logs on startup timeout

[drew]

Summary

Dump recent local gateway diagnostics when the install script cannot connect to the package-managed gateway after startup.

Related Issue

None

Changes

• Add a shared failure diagnostics path for local gateway startup waits.
• Print Homebrew service status and gateway stdout/stderr logs on macOS.
• Print user systemd status and recent journal entries on Linux.
• Allow log tail length override with OPENSHELL_INSTALL_LOG_LINES.

Testing

• mise run pre-commit passes
• Unit tests added/updated
• E2E tests added/updated (if applicable)

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)