Journal writer lock on Windows #1299

Priority: low. Tracked from the OMP pivot plan (ADR-0042, PR #1297), round 3.

Problem

Raxol.Agent.Journal.FileStore.Writer takes its per-session lock only on Unix with a kill binary (writer.ex:782-784). On Windows it runs lockless (writer.ex:717-722), so two BEAMs writing the same session directory produce duplicate offsets and damage the journal (writer.ex:21-24).

Decision for now

P4a of the OMP pivot plan refuses agent_spawn on Windows with a stated reason (fail closed). App tools and the rest of raxol mcp keep working on Windows.

What closing this needs

• A Windows lock primitive for writer.lock. Candidates: an exclusive share-mode open held for the Writer's lifetime, or O_EXCL create plus a process-handle liveness check in place of kill -0.
• A Windows CI leg that exercises it: a second opener is refused while the first lives, and a stale lock left by a killed process is reclaimed.
• Then lift the agent_spawn refusal on Windows.

Manual testing

• On Windows, two raxol mcp processes: agent_spawn c1 in both, and the second is refused
• Kill the first process, then spawn c1 again: it succeeds and the journal offsets continue