refactor: remove dead descriptor-driven mechanisms (no remaining consumer)

Drop code whose only consumer was a retired runtime: hostBehaviors readers for
agentFileExtension, localTargetIsProjectRoot, sharedHooksDirName,
skillsManifestPrefix and skipCodexSkillsManifest; the empty
NON_REGISTRY_CONFIG_HOME_DESCRIPTORS array and live-config-guard plumbing; the
empty RUNTIME_NOTE_AUDIENCE_BY_HEADING filter; the unused resolveVersionFrom
export; and the WINDSURF_SESSION_ID workstream session key. Delete tests that
only exercised those mechanisms.

tests/installer-migration-install.integration.test.cjs

@@ -203,10 +203,7 @@ function assertHasMsdDirectory(root, relPath) {
 function assertFreshInstallContract(runtime, targetDir) {
   const contract = RUNTIME_INSTALL_CONTRACTS[runtime];
   assert.ok(contract, `missing runtime install contract for ${runtime}`);
-  // #3023: the shared hooks bundle's staged directory name is per-runtime
-  // (hostBehaviors.sharedHooksDirName) — resolve it the same way the installer
-  // does rather than hardcoding 'hooks', which is only the default.
-  const hooksDirName = installModule.resolveSharedHooksDirName(runtime);
+  const hooksDirName = installModule.SHARED_HOOKS_DIR;
 
   assert.equal(
     fs.readFileSync(path.join(targetDir, 'msd-core', 'VERSION'), 'utf8'),