refactor(ai-gateway): extract reusable package foundation

[kilo-code-bot]

Summary

• Add a new @kilocode/ai-gateway workspace package as the first framework-neutral extraction point for reusable gateway helpers.
• Move pure gateway config, model ID normalization, kilolog hashing, HTTP response helpers, scheduler/cache adapter types, and no-op observability contracts into the package.
• Rewire web app imports for the extracted helpers while keeping compatibility re-export files under apps/web/src/lib/ai-gateway.

Verification

N/A

Visual Changes

N/A

Reviewer Notes

• This is intentionally a foundation PR, not a full gateway extraction; Next, DB, Redis, usage billing, BYOK, and app-specific orchestration remain in apps/web.
• Automated checks run locally: pnpm --filter @kilocode/ai-gateway typecheck, pnpm --filter @kilocode/ai-gateway lint, pnpm --filter web typecheck, and pnpm format.
• Local commands reported the repo's existing Node engine warning because this environment uses Node v20.20.2 while the repo expects Node >=24 <25.