Fix test imports for Deno 2.6.1 compatibility

[Copilot]

Deno 2.6.1 upgrade broke tests due to @std/assert API changes. Test files imported from "assert" (mapped to v1.0.6) which no longer exports the required assertion functions.

Changes:

• Updated test imports from "assert" to "@std/assert" (v1.0.14)
• Fixed AssertionError constructor to use options object instead of string parameter

Affected files:

• main.test.ts
• src/commands/set.test.ts
• src/commands/options.test.ts
• src/hooks/post.test.ts
• src/util/increment.test.ts
• src/util/version.test.ts

Example change:

// Before
import { assertEquals, AssertionError } from "assert";
const err = new AssertionError("testing");

// After
import { assertEquals, AssertionError } from "@std/assert";
const err = new AssertionError({ message: "testing" });

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