fix: handle StaticSitesClient stderr and non-zero exit codes

[LongOddCode]

Problem

The swa deploy command silently ignores failures from the StaticSitesClient binary. When the binary crashes (e.g., missing native dependencies on node:22-slim Docker images), the CLI reports success and exits with code 0. This causes CI/CD pipelines to mark deployments as succeeded when no content was actually uploaded.

Root Cause

Three bugs in src/cli/commands/deploy/deploy.ts:

1. No stderr handling — child.stderr is never read; all binary errors are silently discarded
2. No non-zero exit code handling — child.on("close") only handles code === 0; no else branch
3. No exit code propagation — CLI never calls process.exit(1) on failure; CI sees exit 0

Fix

• Adds child.stderr.on("data", ...) handler to capture and log binary errors
• Adds else branch for non-zero exit codes with a diagnostic message suggesting ldd to check for missing shared libraries
• Calls process.exit(1) on failure so CI/CD pipelines correctly detect deployment failures

Impact

This is a known issue since July 2022 affecting multiple CI/CD platforms:

• Fixes GitLab CI/CD deployment with SWA does not execute properly #536 (GitLab CI/CD)
• Fixes Azure Static Web app deployes locally but not in CI/CD CircleCI #594 (CircleCI)

Testing

• tsc --noEmit passes
• Changes are isolated to the error handling path; no impact on successful deployments

[LongOddCode]

Closing fork-based PR. Reopening as direct branch PR for CI with secrets.