Windows: preserve native PATH for Codex ACP setup

[paritosh921]

Summary

• Preserve the inherited Windows PATH while probing and launching managed agent commands.
• Allow the codex-acp.cmd npm shim to find standard Node.js installations such as C:\Program Files\nodejs.
• Make the affected agent-environment tests use a Windows-compatible environment-dump command.

Root cause

On Windows, Buzz avoids using the POSIX-style path returned by Git Bash. That also omitted the app's native PATH, so codex-acp.cmd could not resolve node.exe during the adapter version probe and a current adapter was reported as outdated.

Validation

• cargo fmt --all -- --check
• cargo test -p buzz-dev-mcp
• cargo clippy -p buzz-dev-mcp --all-targets -- -D warnings
• Desktop Rust test suite
• pnpm typecheck
• pnpm test
• pnpm build
• Manual Windows test: Codex setup completed without the outdated-adapter message.

Fixes #2342

[Bortlesboat]

I tested this on Windows 11 at 3e0a74c.

One mechanical blocker: cargo fmt --manifest-path desktop/src-tauri/Cargo.toml --all -- --check currently exits 1 at desktop/src-tauri/src/managed_agents/runtime/path.rs:122. The root cargo fmt --all -- --check listed in the PR validation does not cover the excluded desktop crate. Please run the manifest-specific formatter and commit its one-line reflow.

Otherwise the behavior looks correct:

• Current main reproduced the Windows test-harness baseline (15 passed, 2 failed because the tests launch Unix env); this branch passes that slice 17/17 with cmd /C set.
• The full desktop library suite passes on the PR head (1,440 passed, 0 failed, 9 ignored).
• A clean synthetic merge with current origin/main passes (1,469 passed, 0 failed, 9 ignored).
• I also ran a temporary process-level Windows regression: an absolute codex-acp.cmd shim failed to resolve node with a scrubbed PATH, then returned adapter major version 1 when this patch supplied the inherited native PATH.

Passing the inherited PATH explicitly into build_augmented_path is a good shape here: it keeps the behavior testable without mutating global process state and reaches both the CLI-probe and managed-agent launch paths.