docs: ADR 0022 — PEXEX CI/CD efficiency

[samayer12]

Summary

• Adds ADR 0022 proposing changes to the PEXEX E2E test infrastructure to reduce CI cost and improve debuggability
• Continues the lineage of ADR 0009 (created PEXEX) and ADR 0012 (promoted PEXEX to the PR pipeline)
• Documents five proposed changes: npm caching, selective test execution, failure diagnostics, retry transparency, and upstream cache verification
• Documents investigated-and-ruled-out alternatives: shared k3d clusters, build artifact caching, Docker image pre-caching

Related Issues

• pexex: add npm dependency caching to E2E workflow #3076 — npm dependency caching
• pexex: implement selective test execution based on changed paths #3077 — selective test execution
• pexex: capture failure diagnostics in E2E CI #3078 — failure diagnostics
• pexex: surface flaky tests via retry transparency #3079 — retry transparency
• pexex: verify npm cache effectiveness in pepr-triggered PEXEX workflows #3080 — upstream cache verification

Test plan

• ADR content reviewed for accuracy against current PEXEX and pepr CI configuration
• No code changes — documentation only

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