fix(deps): bump qs override to >=6.15.2 to patch DoS (CYPACK-1269)

[cyrusagent]

Summary

Addresses the sole open Dependabot advisory (#140) — a medium-severity remotely-triggerable DoS in qs <6.15.2 (GHSA-q8mj-m7cp-5q26): qs.stringify crashes with a TypeError on null/undefined entries in comma-format arrays when encodeValuesOnly is set.

• qs is pulled in transitively via apps/cli > express@5.2.x > body-parser > qs. Per the team's dependency security policy, this is a deep transitive whose owning direct dep (express) has no released version that resolves qs >=6.15.2, so the existing root pnpm.overrides entry is bumped from >=6.14.2 (which resolved to the still-vulnerable 6.15.1) to >=6.15.2.
• pnpm audit now reports zero advisories.

This supersedes #1251 (which bundled unrelated EgressProxy changes); that PR is being closed in favor of this one.

Linear: CYPACK-1269

Test plan

• pnpm install regenerates lockfile with qs@6.15.2 (single resolved version)
• pnpm audit reports no known vulnerabilities
• pnpm build succeeds
• pnpm typecheck succeeds
• pnpm test:packages:run — all packages green