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.

src/install-engine.cts

@@ -789,13 +789,7 @@ function _copyStaged(stagedDir: string, destDir: string, kind: any, configDir: s
     let destName: string;
     if (kind.kind === 'agents') {
       // Agent files already carry the msd- prefix in the source dir.
-      // #2099: descriptor-driven via hostBehaviors.agentFileExtension. No
-      // current runtime declares one, so destName falls back to entry.name
-      // unchanged (byte-parity, #1575 origin comment).
-      const _agentExt = runtime ? _hostBehaviors(runtime).agentFileExtension : undefined;
-      destName = _agentExt
-        ? entry.name.replace(/\.md$/, _agentExt)
-        : entry.name;
+      destName = entry.name;
     } else {
       // Commands: filename composition (namespacedByDir ? `${stem}.md` :
       // `${prefix}${stem}.md`) is single-sourced with resolveTriggerSurface's
@@ -1919,7 +1913,7 @@ function installOpencodeFamilyArtifacts(
     homeDir: posixNormalize(os.homedir()),
     // #4377: the runtime's own localConfigDir, so an opted-in local install
     // emits `<dir>/...` instead of this checkout's absolute path.
-    localDirName: runtimeArtifactConversion._localIncludeDirName(runtime),
+    localDirName: getDirName(runtime),
   });
 
   // #2329: destDir is derived from the SAME hostBehaviors.flatCommandDir