Export tck.RunE2EKit so other kit repos can reuse the real-sandbox e2e layer

[jedp-docker]

Summary

• TestE2EKit's real-sandbox e2e logic lived entirely inside tck/e2e_test.go, a _test.go file — Go never compiles those into what an importing module gets, so nothing outside this repo could reuse the real-sandbox assertions the way tck.NewSuiteFromDir/RunAll already let importers reuse the container-based TCK layer.
• Moves that logic into an exported tck.RunE2EKit(t, kitPath, tck.E2EOptions) in an ordinary tck/e2e.go, parameterized by an app-name (previously hardcoded to this repo's own sbx-kits-contrib-tck) so a caller can scope every sbx invocation to its own daemon/credential store.
• tck/e2e_test.go's TestE2EKit becomes a thin wrapper — same behavior, same KIT_UNDER_TEST env var contract.
• Documents the new export in README.md's Packages section.
• Motivation: enables other kit repos (e.g. sbx-kits-internal) to import this validator wholesale for their own CI instead of duplicating it, so kit validation elsewhere can't drift from this repo's.

Test plan

• go build ./... and go build -tags=e2e ./... pass
• go vet ./... and go vet -tags=e2e ./... pass
• go test ./spec/... ./tck/... pass
• Live TCK run against a real kit (./scripts/test-kit.sh aider) passes end to end, confirming the refactor didn't change behavior
• CI: confirm e2e-release/e2e-nightly still pass on a real kit change

Known CI failures (pre-existing, unrelated to this change)

This PR touches tck/, which triggers detect-changes' "run every kit" rule — the first full-repo TCK sweep in a while. Two kits fail, both due to floating external references unrelated to anything in this diff:

• aidlc-claude — install step does git clone --branch v2 ... against awslabs/aidlc-workflows, which has renamed its active branch to main. Already fixed in fix(aidlc-claude): track aidlc-workflows main instead of v2 #267 (open); once that merges, this failure should disappear here too.
• claude-mem — install step (npx -y claude-mem@latest install) now fails non-interactively with provider-selection failed during non-interactive-validation: A provider must be explicit when stdin is not interactive. This looks like a recent upstream claude-mem@latest release added a new interactive provider-selection prompt. No fix PR found for this one yet.

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