perf(r2): reuse validated S3 client

[vsolano9]

Closes #45

What changed

• lazily create and memoize one Cloudflare R2 S3Client for upload and read presigns
• validate all four required R2 variables before client construction and name the missing variable in the error
• reuse the validated bucket name instead of repeated non-null assertions
• cover client reuse and every required-variable failure without contacting R2

Validation

• Node 22.23.2: npx vitest run — 14 files, 126 tests passed
• Node 22.23.2: npx tsc --noEmit
• Node 22.23.2: npm run lint — no warnings or errors
• Node 22.23.2: npx prettier --check .
• Node 22.23.2: npm run build with the CI placeholder environment
• focused R2 test file repeated 20 times

No dependency or lockfile changes.

[github-actions]

Welcome, @vsolano9, and thanks for your first pull request to agentpostmortem.

A quick look at CONTRIBUTING.md covers how things are set up and run here.

I'll review this shortly. Ask anything in the thread if something is unclear.