docs(conformance): route agents to portable test guide

[elezar]

Summary

Make the installed-artifact conformance suite README the canonical guide for deciding when portable conformance coverage is required and where OpenShell tests should live. Route contributor agents to that guide so conformance changes are not missed merely because e2e/ was unchanged.

Related Issue

No issue required: this is scoped documentation and internal agent-workflow maintenance.

Changes

• Document the boundary between portable conformance, driver, feature, E2E, and crate-local tests.
• Document the shared scenario library, installed-artifact workspace, scenario authoring flow, cleanup requirements, and focused Cargo verification commands.
• Add a concise conformance-routing rule to AGENTS.md.
• Update build-from-issue, create-spike, and review-github-pr to consult the canonical guide without duplicating its detailed rules.
• Expand build-from-issue verification triggers to cover conformance library and installed-suite changes even when e2e/ is unchanged.
• Add conformance routing to the sync-agent-infra maintenance map and consistency checklist.

Testing

• mise run pre-commit passed before the final review-driven README deletion; a subsequent full rerun reached a host sccache: Operation not permitted error during lockfile validation.
• mise run markdown:lint:md passed after the final README edits.
• cargo test -p openshell-conformance -p openshell-conformance-cli --no-run passed with RUSTC_WRAPPER disabled.
• cargo test --manifest-path tests/suites/conformance/Cargo.toml --no-run passed with RUSTC_WRAPPER disabled.
• Agent Skills CLI discovery found exactly the four public skills.
• E2E tests added/updated (not applicable: documentation and agent guidance only).

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)