fix(desktop): fix Windows PATH clobber and .cmd shim EINVAL

[wpfleger96]

What

Three interacting defects caused Claude Code and Codex installs to fail on Windows with 'npm: command not found', installed adapters to appear missing, and agent turns to EINVAL-fail.

1. Install PATH clobber (install_shell_command)

install_shell_command() built PATH from managed-Node dirs + login_shell_path(), but both are always None on Windows: no managed Node runtime for Windows exists yet, and login_shell_path() returns None because Git Bash returns POSIX colon-delimited paths that poison native children. With an empty path_parts the PATH was unset entirely, so the Git Bash install shell couldn't find npm → install failed.

2. Probe/runtime PATH clobber (build_augmented_path)

Same omission in build_augmented_path(). Callers pass its result to Command::env("PATH", …) which replaces rather than extends the child's PATH. Without the inherited process PATH every readiness probe, version probe, and launched agent lost node/npm/git — installed adapters appeared missing and agent turns failed with 'node' is not recognized.

3. .cmd shim EINVAL (configure_runtime_cli)

configure_runtime_cli() resolved the Claude CLI and set CLAUDE_CODE_EXECUTABLE unconditionally. On Windows resolve_command returns claude.cmd (an npm shim); passing that path to CreateProcess causes EINVAL — batch scripts cannot be exec'd directly by Node.

Fix

is_batch_shim(path: &Path) -> bool — pure host-independent predicate in runtime/path.rs. Returns true for .cmd/.bat extensions (case-insensitive). Used internally by should_skip_claude_executable and tested directly via super::path::is_batch_shim in runtime/tests.rs.

should_skip_claude_executable(path: &Path, is_windows: bool) -> bool — pure policy predicate in runtime/path.rs, re-exported from runtime.rs. On Windows, skips .cmd/.bat paths for CLAUDE_CODE_EXECUTABLE so the adapter falls back to its own PATH lookup. On non-Windows, .cmd/.bat are valid executables and must be assigned — the explicit is_windows flag restores that behavior and makes the OS gate testable cross-host on macOS CI. Production call: should_skip_claude_executable(&cli_path, cfg!(windows)).

should_use_inherited(had_shell_path, has_local_context, is_windows) -> bool — pure policy predicate in runtime/path.rs, re-exported from runtime.rs. Both build_augmented_path and install_shell_command call it — the inherited-PATH policy lives in exactly one place and is testable cross-host.

compose_path_entries(managed, login, inherited, use_inherited) -> Vec<PathBuf> — pure PATH composition kernel (pub(crate)) in runtime/path.rs, called by both build_augmented_path and install_shell_command. Eliminates the drift hazard between the two paths. Inputs are already-split Vec<PathBuf>; split_paths/join_paths stay at wrapper boundaries.

Non-Windows impact

None. should_use_inherited returns false when is_windows=false; should_skip_claude_executable returns false when is_windows=false. Both use cfg!(windows) at their production call sites, which is a compile-time constant — the Windows branches are dead-code-eliminated on non-Windows. Unix output is byte-identical to before.

Tests

Pure, host-agnostic (run on all CI targets including macOS):

• runtime/tests.rs: 6 is_batch_shim predicate tests via super::path::is_batch_shim
• runtime/path.rs compose_tests: should_use_inherited truth table (4 cases), compose_path_entries ordering, empty/unset inherited with Windows policy ON, should_skip_claude_executable policy matrix (4 tests covering shim+Windows→skip, shim+non-Windows→assign, exe/no-ext both ways)

Platform-gated:

• runtime/path.rs #[cfg(unix)]: unix_shell_path_output_unchanged_by_windows_fallback_logic
• runtime/path.rs #[cfg(windows)]: process-PATH append, shell-PATH-present guard, no-context guard
• agent_discovery.rs #[cfg(windows)]: test_install_shell_command_includes_process_path_on_windows

Fixes #2327, Fixes #2342, Fixes #2397
Supersedes #2247, #2420, #2506, #2533 (PATH family), #2495 (.cmd shim)