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/plan-pre-hook-e2e.test.cjs

@@ -53,7 +53,6 @@ const CLEAN_ENV = {
   OPENCODE_SESSION_ID: '',
   GEMINI_SESSION_ID: '',
   CURSOR_SESSION_ID: '',
-  WINDSURF_SESSION_ID: '',
   TERM_SESSION_ID: '',
   WT_SESSION: '',
   TMUX_PANE: '',