fix(memory): EverOS memory unavailable on native Windows

[Kendrick-Song]

Problem

EverOS (everos/core/persistence/locking.py:13) imports fcntl at module top, which is POSIX-only. On native Windows, everos server start crashes with ModuleNotFoundError: No module named 'fcntl'.

Current state (after #170)

• Raven side: platform guard added -- onboard skips memory config on win32, backend.start() degrades to NoOpAdapter with a warning. Users are guided to WSL.
• EverOS side: upstream declares "Windows is not supported" in docstring. The win_fcntl_shim in Raven's embedded mode used to paper over this, but HTTP-only mode spawns a child process where the shim has no effect.

Impact

Windows users get no EverOS memory at all (no episodes, profiles, skills, recall). Native Markdown memory (MEMORY.md) still works.

Possible fixes

1. EverOS upstream: replace fcntl with portalocker (or equivalent) in everos/core/persistence/locking.py -- same approach Raven took in raven/utils/portable_lock.py
2. Raven workaround: inject win_fcntl_shim into the spawned subprocess environment before everos server start (fragile, depends on everos internals)
3. Accept and document: Windows users use WSL (current state)

Option 1 is the clean fix but requires an everos release.

Evidence

• everos==1.1.3, everos/core/persistence/locking.py:13: import fcntl
• Raven guard: raven/plugin/memory/everos/backend.py (win32 check in start())
• Raven guard: raven/cli/onboard_commands.py (win32 check in step 4)