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/injection-patterns-parity.security.test.cjs

@@ -36,7 +36,7 @@ const READ_SCANNER_HOOK = path.join(__dirname, '..', 'hooks', 'msd-read-injectio
 // per-index assertion fails if a pattern is edited past its sample.
 // Built from fragments (never a complete phrase in source) so this file itself
 // does not trip the CI prompt-injection scanner, which scans the PR diff —
-// same technique as tests/shared-hooks-dir-resolution.test.cjs (#3175). The
+// same technique as the other injection-pattern tests (#3175). The
 // assembled runtime strings are still real payloads both hooks must catch.
 const SAMPLES = [
   // #4016: index 0 is the single filler-tolerant imperative-override pattern