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.

scripts/lib/macos-conformance-tier.generated.cjs

@@ -185,7 +185,6 @@ module.exports = {
   "tests/runtime-name-policy.test.cjs",
   "tests/security.test.cjs",
   "tests/settings-jsonc.test.cjs",
-  "tests/shared-hooks-dir-resolution.test.cjs",
   "tests/shell-command-projection-dispatch.test.cjs",
   "tests/spawn-liveness-banner.test.cjs",
   "tests/state-document.test.cjs",

scripts/lib/platform-conformance-tier.generated.cjs

@@ -234,7 +234,6 @@ module.exports = {
   "tests/security.test.cjs",
   "tests/settings-jsonc.test.cjs",
   "tests/sh-hook-paths.test.cjs",
-  "tests/shared-hooks-dir-resolution.test.cjs",
   "tests/shell-command-projection-dispatch.test.cjs",
   "tests/shell-command-projection-path-sep.test.cjs",
   "tests/ship-notes-wedged-pr.test.cjs",