Fleet Tests Run on Stable/Main Despite Being Experimental — Missing Feature Gate + Platform Guard #2257

Problem

Fleet is a develop-only experimental feature, but test_orphan_l3_reaping (and potentially other fleet tests) ran on the macOS-15 CI runner when PR #945 targeted stable. Two gaps:

1. Feature gate gap: Fleet tests are not excluded by feature flag when running on stable/main branches. As an experimental feature, fleet components should not be tested (or accessible) outside develop.

2. Platform guard gap: Fleet tests that depend on Linux-specific infrastructure (/proc via read_starttime_ticks, read_boot_id) lack sys.platform != "linux" skip markers.

Expected Behavior

• Fleet tests should be deselected on stable/main CI runs via feature flag / experiment type gating
• Fleet tests that depend on /proc-based identity (_linux_proc.py) should carry @pytest.mark.skipif(sys.platform != "linux", ...) as a secondary guard
• The fleet feature should not be accessible (components, tools, CLI subcommands) when running on stable/main

Affected Test Files

At minimum: tests/fleet/test_fleet_e2e.py::test_orphan_l3_reaping

A broader audit of tests/fleet/ should identify all tests that:

• Use read_starttime_ticks() or read_boot_id() (Linux-only)
• Test fleet reaping, liveness, or dispatch lifecycle that touches /proc

Existing Precedents

Other Linux-only test modules are properly gated:

• tests/execution/test_linux_tracing.py — module-level pytestmark = [pytest.mark.skipif(sys.platform != "linux")]
• tests/execution/test_session_log_integration.py — same pattern
• tests/fleet/test_liveness.py — uses pytest.skip("Not on Linux") inside individual tests

Context

• Discovered via PR test #945 macOS-15 CI failure
• Investigation report: .autoskillit/temp/investigate/investigation_macos_ci_failures_pr945_2026-05-08_160000.md
• Companion issue for the reaper's /proc dependency: see fleet reaper identity check issue