refactor(#1511): move content-rewrite engine to conversion module, delete the install.js relay (#1513)

* refactor(#1511): move content-rewrite engine to conversion module, delete the install.js relay

Phase 2 of epic #1507 (ADR-1508). Behavior-preserving: makes the Runtime
Artifact Conversion Module the single owner of per-runtime content rewriting and
removes the last upward .cts -> bin/install.js dependency.

- src/runtime-artifact-conversion.cts now owns the engine (_applyRuntimeRewrites,
  5-arg with INJECTED attribution), the staged-content walkers
  (applyRuntimeContentRewritesInPlace / ...ForCommandsInPlace), computePathPrefix
  (private, exported as _computePathPrefix for tests), and the deep public seam
  rewriteStagedSkillBodies / rewriteStagedCommandBodies({runtime, configDir,
  scope, homedir?, platform?, resolveAttribution?}).
- src/surface.cts:applySurface calls rewriteStagedSkillBodies directly (no
  resolveAttribution -> undefined). Co-Authored-By is absent from ALL rewritten
  content, so processAttribution is vacuous there and undefined is provably
  behavior-identical. surface no longer imports getInstallExports.
- src/runtime-artifact-layout.cts: deleted getInstallExports / loadInstallExports
  / InstallExports + the GSD_TEST_MODE require('bin/install.js') relay.
- bin/install.js: binds computePathPrefix / the two walkers / _applyRuntimeRewrites
  from the conversion module (single implementation, exports preserved for Hyrum);
  install callsites pass getCommitAttribution(runtime) as the injected attribution.
  getCommitAttribution stays here (impure install-time config I/O).
- DEFECT.GENERATIVE-FIX guard: tests assert install.X === conversion.X reference
  identity for computePathPrefix + both walkers (no drift).

New tests/enh-1511-*.test.cjs (engine, attribution injection, deep seam, prefix,
layout-no-relay guard, reference-identity). 316 affected-suite tests green; lint clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0187qgypdy1wkWRpdaf2hRuD

* test(#1511): make rewrite-engine path assertions Windows-robust

The deep seam normalizes paths as path.resolve(configDir).replace(/\\/g,'/')
and compares homedir().replace(/\\/g,'/'). Three assertions in the new test
rebuilt expected paths without that normalization, so they passed on Mac/Linux
but failed on Windows CI (PR #1513):

- two absolute-branch asserts rebuilt resolvedTarget via path.resolve(configDir)
  without the backslash→slash replace → mismatch on Windows.
- the $HOME-branch test fed a POSIX-literal /home/testuser, which Windows
  path.resolve re-roots onto the cwd drive (D:/home/...), so the
  resolvedTarget.startsWith(homeDir) check failed and the $HOME shorthand was
  never produced.

Fix is test-only (engine unchanged, still behavior-preserving): mirror the
engine's .replace(/\\/g,'/') in the two absolute-branch asserts, and use a real
absolute path (path.resolve(os.tmpdir(), ...)) + platform: process.platform for
the $HOME-branch test so the comparison holds on all platforms.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0187qgypdy1wkWRpdaf2hRuD

---------

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

src/surface.cts

@@ -30,7 +30,6 @@
 
 import fs from 'node:fs';
 import path from 'node:path';
-import os from 'node:os';
 import { platformWriteSync } from './shell-command-projection.cjs';
 // eslint-disable-next-line @typescript-eslint/no-require-imports
 import installProfiles = require('./install-profiles.cjs');
@@ -43,7 +42,9 @@ import { CLUSTERS } from './clusters.cjs';
 import type { ClusterMap } from './clusters.cjs';
 // eslint-disable-next-line @typescript-eslint/no-require-imports
 import runtimeArtifactLayout = require('./runtime-artifact-layout.cjs');
-const { findInstallSourceRoot, getInstallExports } = runtimeArtifactLayout;
+const { findInstallSourceRoot } = runtimeArtifactLayout;
+// eslint-disable-next-line @typescript-eslint/no-require-imports
+import runtimeArtifactConversion = require('./runtime-artifact-conversion.cjs');
 
 const SURFACE_FILE_NAME = '.gsd-surface.json';
 
@@ -305,26 +306,18 @@ function applySurface(runtimeConfigDir: string, layout: Layout, manifest: Map<st
   const resolved = resolveSurface(layout.configDir, skillManifest, clusterMap, registry);
   // Mirror installRuntimeArtifacts: skills kinds get per-runtime path rewrites
   // so SKILL.md bodies reference the install target (pathPrefix), not the
-  // converter's default ~/.claude paths (#813). Computed lazily so command-only
-  // runtimes do not trigger the install.js require.
-  let pathPrefix: string | null = null;
+  // converter's default ~/.claude paths (#813). Delegated to the conversion
+  // module's deep seam (ADR-1508 / #1511 Phase 2) — no attribution resolver
+  // needed here (proven: Co-Authored-By never appears in staged content; see
+  // brief PROVEN KEY FACT). No getInstallExports() call required.
   for (const kind of layout.kinds) {
     const staged = kind.stage(resolved);
     if (kind.kind === 'skills') {
-      const installExports = getInstallExports();
-      if (pathPrefix === null) {
-        const scope = layout.scope ?? 'global';
-        const resolvedTarget = path.resolve(layout.configDir).replace(/\\/g, '/');
-        const homeDir = os.homedir().replace(/\\/g, '/');
-        pathPrefix = installExports.computePathPrefix({
-          isGlobal: scope === 'global',
-          isOpencode: layout.runtime === 'opencode',
-          isWindowsHost: process.platform === 'win32',
-          resolvedTarget,
-          homeDir,
-        });
-      }
-      installExports.applyRuntimeContentRewritesInPlace(staged, layout.runtime, pathPrefix);
+      runtimeArtifactConversion.rewriteStagedSkillBodies(staged, {
+        runtime: layout.runtime,
+        configDir: layout.configDir,
+        scope: layout.scope ?? 'global',
+      });
     }
     const dest = path.join(layout.configDir, kind.destSubpath);
     _syncGsdDir(staged, dest, kind, skillManifest);