feat: support WSL relay receivers targeting native Windows Codex #110

Problem

v1.0.0-alpha.1 supports paired-device receivers on Linux/macOS, while the core CLI separately supports native Windows Codex sessions. On a Windows workstation, WSL2 is therefore the supported place to run the relay receiver, but that receiver cannot submit to the workstation's native Windows Codex session without an operator-built wrapper.

A WSL receiver can read the native Codex state DB through a mounted codexHome, so policy validation and relay list succeed. Relay send then returns native_outcome_unknown because:

1. Policy permits codexHome but not codexBin;
2. Linux executable discovery looks for codex, while the installed native binary is codex.exe;
3. the Windows process needs a Windows CODEX_HOME, not /mnt/c/....

The public relay, admission, pairing, inner TLS, and Codex queue all worked after adding a private test wrapper that exposed codex, converted CODEX_HOME with wslpath -w, and invoked codex.exe. That wrapper is not suitable as an undocumented user prerequisite.

This is distinct from #22: native Windows core list/send support is complete there. The gap is the relay receiver's WSL-to-native-Windows adapter boundary introduced by #69.

Reproduction

Environment:

• Windows 10 with native Codex CLI 0.154.0 and an existing native Codex session
• Ubuntu 24.04 on WSL2, Python 3.12
• session-peer[relay]==1.0.0a1
• policy target uses agent: codex, a native session target, and /mnt/c/Users/<user>/.codex

Observed:

• local and remote relay list: succeeds;
• relay send: receiver reports native_outcome_unknown;
• direct local dry-run inside WSL gives Codex executable not found: 'codex';
• a private codex wrapper plus CODEX_HOME conversion makes dry-run and real transcript ACK succeed.

No credentials, private keys, device identifiers, session identifiers, or host addresses are included here.

Proposed direction

Choose and document one explicit contract:

• support WSL receivers targeting native Windows Codex by adding a reviewed adapter option that carries an explicit executable and converts the home path safely; or
• reject mounted native Windows homes with a specific error and document a supported architecture that doesn't require an ad-hoc wrapper.

Avoid shell command construction. Preserve the existing policy allowlist, absolute-path validation, fixed argv invocation, and no-secret logging behavior.

Acceptance criteria

• A WSL2 receiver can list and submit to an allowlisted native Windows Codex session without a user-authored wrapper, or fails early with a specific documented unsupported-boundary error.
• The receiver policy has an explicit, validated executable/home contract; arbitrary command strings aren't accepted.
• Windows and POSIX path conversion is covered by unit tests.
• A WSL2 ↔ Linux forced-public-relay integration test covers list, real queue submission, and transcript ACK.
• Existing Linux/macOS relay targets and native Windows core behavior remain unchanged.
• User-facing relay setup docs explain the supported Windows topology.