test(e2e): run model-free e2e tests at PR time; assert --no-sandbox exposure

[nhuelstng]

What

• Add an e2e_fast build tag on the model-free files in internal/e2e/ (//go:build e2e || e2e_fast) and a new E2E fast CI job that runs just that slice on ubuntu-latest + macos-latest on every push/PR.
• Close the --no-sandbox gap: runSecurityAudit no longer silently skips all negative assertions — it asserts the audit ran to completion and documents the per-property exposure surface.
• Refactor the pure assertion predicates and buildOmac into shared, fast-buildable files.

Why

The entire internal/e2e/ package sits behind //go:build e2e, and that tag is compiled only by e2e.yml (weekly cron + manual dispatch). ci.yml and release.yml run go test ./... without it, so the model-free security-regression tests never ran on the merge path — even the ones explicitly built to run fast without a live model:

Test	Guards
TestE2EServeDirTokenIsolation	the real #74 dir_token cross-workdir leak, over real loopback sockets
security_assertions_test.go	assertions silently becoming no-ops (the #84 rigor gap / #66 "mutation-test the tests")
versions_test.go, harnesses_test.go	harness/version registry invariants

Consequence: a reintroduced dir_token leak or a neutered fs-deny assertion would pass CI, pass the release gate, and ship — surfacing up to a week later. This moves the fast net onto the code path that produces releases.

For --no-sandbox harnesses (codex/macOS), the audit previously skipped every negative assertion, silently excluding the real risk surface (#66). It now fails loudly if the audit produced nothing and logs an explicit EXPOSED/contained table per property.

How

• e2e || e2e_fast on harnesses.go, versions.go, classify.go, fixtures.go (new), security_assertions.go (new), and the four model-free test files. The live files (e2e_test.go, allowance.go, artifacts.go, provenance_test.go) stay e2e-only.
• Full live suite is unchanged: those files build under both tags, so e2e.yml's -tags=e2e still runs everything weekly.
• noSandboxExposureReport is a pure function unit-tested in the fast slice (TestNoSandboxExposureReport), in the repo's "mutation-test the tests" spirit.
• No bubblewrap / token / harness install in the fast job — omac serve --no-inner is control-plane only.

Verification

go test -tags=e2e_fast -race ./internal/e2e/   # 11 tests pass (~3s), incl. real serve subprocess
go vet  -tags=e2e       ./internal/e2e/         # full live suite still compiles
go build ./... && go vet ./...                  # untagged build unaffected

Follow-up

A separate release-procedure gap (release currently gates only on untagged go test) will be filed as its own issue.

Refs #66

🤖 Generated with Claude Code

[nhuelstng]

Follow-up release-procedure gap filed as #101 (release currently gates only on the untagged go test; the e2e_fast slice added here doesn't yet run before a tag is cut).