Remove unused remote-theme archive dependencies from site builds

[valzargaming]

The local Hyde site has no remote-theme configuration or archive input, but the blanket github-pages development metagem installs jekyll-remote-theme and an affected rubyzip version that cannot upgrade under its <3 constraint. Replace that unused dependency chain with explicit Jekyll 3.10.0, the existing plugins, and GFM parser. Legacy master-based Pages hosting is unchanged.

Adds a pre-render dependency/configuration guard with negative controls, corrects the alternate-origin fixture working directory for Jekyll 3.10, and makes the Civ14 banner path explicitly base-path aware. Bundler generated the reduced lockfile; temporary resolution steps were removed.

Validation at f14642d:

• Frozen-lock Ruby 3.3/Bundler 2.7.2 build run 38146965401 passed, including dependency guard, remote-theme negative controls, all preview fixtures, and companion banner base-path assertion. All CodeQL checks passed.
• Exact-head artifact passes local preview and static checks (5 pages, no missing local links/assets). Generated homepage/companion output was compared to the prior verified artifact; content preserved, with renderer version/whitespace differences and explicit banner path.
• Browser artifact tests passed for filtering, gallery open/Escape focus return, Civ14 navigation, no horizontal overflow, and no console errors. Temporary tab and server closed.
• Independent boundary investigation and candidate review found no remaining repository-managed archive extraction path. Vulnerability substitute verifies absence of rubyzip/remote-theme in the resolved bundle and rejects both legacy/current configuration activation forms.

Scope: removes affected repository-managed build dependency. GitHub manages the legacy Pages dependency environment separately; this site supplies no remote-theme archive there. This is not a claim to patch GitHub's service or reproduce an exploitable public endpoint.

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