ci(os-49): remove obsolete shadow PR workflows

[jtoelke2]

Summary

• Remove the Phase 5 shadow PR workflows that still triggered on pull-request/* after the Phase 6 cutover.
• Keep reusable/manual shadow workflows that are still useful, including shadow-rust-native-build.yml, shadow-docker-build.yml, and shadow-shared-cpu-spike.yml.
• Update CI architecture docs to describe the post-cutover state.
• Refresh mise.lock with mise lock so the new lockfile check passes on the current tool metadata.

Related Issue

• OS-49

Changes

• Delete shadow-branch-checks.yml, shadow-branch-e2e.yml, and shadow-ci-image.yml.
• Remove stale docs references to the deleted PR-triggered shadow workflows.
• Update Python standalone lockfile entries from the current mise lock output.

Testing

• git diff --check passed.
• Confirmed no stale references remain with rg.
• mise lock was run with network access and matches the CI lockfile diff.
• mise run pre-commit was run; it failed in existing Rust test ssh::tests::launch_editor_returns_friendly_error_when_binary_missing, unrelated to these workflow/doc/lockfile changes. The single test reproduces locally with mise x -- cargo test -p openshell-cli ssh::tests::launch_editor_returns_friendly_error_when_binary_missing -- --nocapture.

Checklist

• Commit messages follow Conventional Commits.
• Scope limited to Phase 6 shadow workflow cleanup plus required lockfile refresh.
• Documentation updated for the workflow removal.