test(language/lint): add unit tests for unreachable-code pass

[dsorajisto]

What

Adds 5 unit tests for the unreachable-code lint pass in packages/language/src/lint/unreachable-code.ts. Brings the rule's test count from 0 to 5.

Why

The unreachable-code rule flags code that follows a terminal statement — either a bare transition or an if/else block where all branches transition. It had no tests pinning down its behavior. This closes the gap using the same pattern PRs #38 (required-fields.test.ts), #48 (empty-block.test.ts), and #60 (unused-variable.test.ts) established for single-rule lint tests.

How

New test file at packages/language/src/lint/unreachable-code.test.ts using vitest. Follows unsupported-conditionals.test.ts exactly: single describe block, flat it cases, shared getDiagnostics helper, and the same ProcBlock / ProcedureValue schema shape so the procedural fixtures parse correctly.

The 5 cases:

1. Flags code after a transition
2. Does not flag a procedure whose last statement is a transition
3. Flags code after an if/else where both branches transition
4. Does not flag code after an if/else where only one branch transitions
5. Flags unreachable code inside a nested if body

Test Plan

pnpm --filter @agentscript/language test unreachable-code

Result:

✓ src/lint/unreachable-code.test.ts (5 tests) 7ms
Test Files  1 passed (1)
     Tests  5 passed (5)

• pnpm --filter @agentscript/language test unreachable-code — 5/5 passing
• npx eslint --config eslint.config.js packages/language/src/lint/unreachable-code.test.ts — exit 0
• New/updated tests cover the change — N/A (this PR is the tests)

Checklist

• My code follows the project's coding style
• I have reviewed my own diff
• I have added/updated documentation as needed
• This change does not introduce new warnings