refactor(#2246): centralize cross-platform path-separator handling (toPosixPath / toNativePath / posixNormalize) (#2247)

Replace every open-coded separator translation across the installer/hooks
source with named, tested seams in shell-command-projection.cts (the platform
seam), removing all hardcoded `/`+`\` from path handling:

- toPosixPath(p)   — this machine's native path → POSIX (running-OS relative;
                     for local filesystem paths).
- toNativePath(p)  — POSIX → native (collapses the win32 `/\//g,'\\'` ternary).
- posixNormalize(p)— unconditional `\`→`/`, OS-independent; for emitting paths
                     to a POSIX/bash TARGET (which may differ from the running
                     OS) and for parsing mixed-separator input.

core-utils.toPosixPath now delegates to the seam, so its 20+ existing consumers
resolve to one implementation; no duplicate helper.

- ~47 sites across runtime-hooks-surface, runtime-artifact-conversion,
  runtime-artifact-install-plan, drift, init, worktree-safety,
  installer-migrations, installer-migration-authoring, install-engine, surface,
  verify, runtime-artifact-layout, schema-detect, check-command-router.
- Closes the latent POSIX-literal-backslash corruption class (the regex form
  corrupts a POSIX path containing a literal backslash; split(path.sep) does not).
- New unit + fast-check property tests for all three helpers.

Closes #2246

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/surface.cts

@@ -31,7 +31,7 @@
 import fs from 'node:fs';
 import os from 'node:os';
 import path from 'node:path';
-import { platformWriteSync } from './shell-command-projection.cjs';
+import { platformWriteSync, posixNormalize } from './shell-command-projection.cjs';
 // eslint-disable-next-line @typescript-eslint/no-require-imports
 import installProfiles = require('./install-profiles.cjs');
 const {
@@ -354,8 +354,8 @@ function applySurface(runtimeConfigDir: string, layout: Layout, manifest: Map<st
   // surface-path agents lack path-prefix rewrites and Co-Authored-By trailers,
   // diverging from a fresh install.
   const _homedirFn: () => string = opts?.homedir ?? (() => os.homedir());
-  const _resolvedTarget = path.resolve(layout.configDir).replace(/\\/g, '/');
-  const _homeDir = _homedirFn().replace(/\\/g, '/');
+  const _resolvedTarget = posixNormalize(path.resolve(layout.configDir));
+  const _homeDir = posixNormalize(_homedirFn());
   const _isGlobal = (layout.scope ?? 'global') === 'global';
   const _isOpencode = layout.runtime === 'opencode';
   const _isWindowsHost = (opts?.platform ?? process.platform) === 'win32';