Add release packaging tools and refreshed icons

[patrickschiller]

What changed

• add the reusable SourceBraid liquid-glass icon master and regenerate the Chrome, plugin, and iOS icon assets
• add explicit-allowlist builders for the Chrome extension and skills-only Codex plugin packages
• add a guarded GitHub setup helper for private SourceBraid archives
• add tests that enforce package allowlists, reject symlinks, preserve existing setup files, and refuse public archive repositories

Why

This makes release artifacts reproducible, keeps SourceBraid branding consistent across surfaces, and gives users a safer path for initializing their private archive repository.

Privacy and permissions

• no browser permission, GitHub scope, network endpoint, or archive format changes
• no private website, captured archive content, credentials, or private Git history are included
• release builders package only explicit allowlists
• the setup helper requires an existing authenticated GitHub CLI session, refuses public target repositories, uploads only known support files, and does not persist tokens

Checks

• node --test tests/capture-utils.test.js — 15 passed
• python3 -m unittest discover -s tests -p "test_*.py" — 34 passed
• unsigned iOS Simulator build — succeeded
• Chrome extension allowlist package build — succeeded
• skills-only Codex plugin package build — succeeded