feat(allowlist): add Zig support

[nightcityblade]

Problem Statement

OpenCodeReview does not currently recognize Zig source files, so .zig changes are skipped by the review allowlist. This is the Zig language-group sub-issue requested by #470.

Proposed Solution

• Add .zig to the supported file extensions.
• Exclude conventional Zig tests at **/test/*.zig and **/*_test.zig.
• Add focused tests for both extension recognition and test-path exclusion.

Alternatives Considered

Users could customize allowlists locally, but built-in support gives Zig projects the same default behavior as the other supported languages.

Affected Area

• Configuration

Additional Context

Tracking issue: #470

[nightcityblade]

Hi, I'd like to work on this. I'll submit a focused PR shortly.