Align README platform and containment-test guidance with current code #728

Summary

Installation and verification guidance contradict current portable read paths and the Linux CI wrapper.

Evidence

On main fa0c79b9ae38eca2c59b2b1911becfe1a6b3c9ab:

• README.md:1182-1186 says execution and repository mapping require Linux with a delegated systemd manager. The platform section at lines 2111-2117 separately describes portable Git/process/state operations.
• src/repo_map.rs:64-80, src/safe_state.rs:1002-1103 and src/repo_semantic.rs:648-667 include portable binding/read/traversal paths restored under repository-file reads are Linux-only: repo map fails on macOS and Windows, and repo_semantic's cfg(unix) gate strands macOS with no fallback #190. Full native command runtime qualification was not performed in this audit, so the documentation should name the actual supported surface and preserve remaining unverified limits.
• README.md:1246-1251 says hosted Linux CI runs plain Cargo tests with containment tests skipping when no user manager exists.
• .github/workflows/ci.yml:56-65,97-98,110-119,131-132 and .github/scripts/run-linux-ci-cargo-test-partition.sh:54-59 deliberately execute tests through a delegated-user-scope wrapper. The wrapper verifies its cgroup and launches the partitions (run-linux-all-tests-in-delegated-user-scope.sh:224-253).

Readers can misinterpret green CI as skipped containment coverage and are directed to WSL for operations whose implementations have portable paths.

Acceptance criteria

• Distinguish portable commands from operations requiring Linux containment or Linux-only primitives, and explicitly qualify any native runtime support not yet exercised.
• Explain the current delegated hosted-CI setup and distinguish it from ordinary hosts that actually self-skip.
• Give a correct local parity command with the wrapper's prerequisites, and align nearby install/verification examples.
• Keep compile portability, runtime qualification and full containment acceptance distinct.

Verification and related work

Exact-revision source/documentation comparison; no new platform runtime or full test run. Closed #63/#101/#116/#190 established these implementations. No open Issue/PR updates the contradictory guidance.