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/runtime-artifact-layout.cts

@@ -30,6 +30,7 @@ import runtimeArtifactConversion = require('./runtime-artifact-conversion.cjs');
 const conversionExports = runtimeArtifactConversion as Record<string, unknown> & {
   readGsdCommandNames?: () => string[];
 };
+import { posixNormalize } from './shell-command-projection.cjs';
 
 // In .cts (CommonJS output) files, `require` is available as a global.
 const _require: NodeRequire = require;
@@ -279,7 +280,7 @@ function kimiAgentsKind(destSubpath: string, prefix: string, configDir: string):
           if (!entry.isFile() || !entry.name.endsWith('.md')) continue;
           const agentPath = path.join(stagedAgents, entry.name);
           subagents.push({
-            path: path.join('agents', entry.name).replace(/\\/g, '/'),
+            path: posixNormalize(path.join('agents', entry.name)),
             content: fs.readFileSync(agentPath, 'utf8'),
           });
         }