feat(#2088): migrate Codex onto the Embeddable Orchestration System (ADR-1239)

Drive Codex install/uninstall through the descriptor-driven Host-Integration
Interface (declarative embedding adapter → engine surface dispatch) and fold
every positive `runtime === 'codex'` / `isCodex` projection into descriptor-driven
`runtime.hostBehaviors`. Install/uninstall output stays byte-parity-gated
(tests/fixtures/golden-install-parity/codex.json); no other runtime changes.

Three Context7-verified upgrades, each with a test on the user-reachable surface:
- Skill root → canonical $HOME/.agents/skills via a skills-kind `home` override,
  with pre-move migration cleanup (stale ~/.codex/skills/gsd-* removed on install
  and uninstall; user content preserved). Fixes getGlobalSkillsBase, writeManifest,
  and the skill-manifest inventory to honor the override so --skills-root /
  sync-skills / the manifest report the real location.
- Six new hooks.json lifecycle events (PreToolUse, PermissionRequest, PreCompact,
  PostCompact, SubagentStop, UserPromptSubmit) shared by install + uninstall;
  extendedHookEvents reconciled [] -> the schema-valid wired subset.
- Explicit `[agents] max_depth = 1` in the managed config.toml block, pinning the
  negotiated dispatch.maxDepth:1 axis. validateCodexConfigSchema now permits a
  known-scalar-only bare `[agents]` AgentsToml table (still rejects [[agents]] and
  unknown-key break-forms, #2760); mergeCodexConfig preserves the user's own
  AgentsToml scalars (max_threads etc.) instead of dropping them.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/runtime-artifact-install-plan.cts

@@ -32,6 +32,10 @@ interface ArtifactKind {
   destSubpath: string;
   prefix?: string;
   stage: (resolvedProfile: ResolvedProfile, agentCtx?: AgentCtx) => string;
+  /** Resolved absolute alternate install root for this kind, if the descriptor
+   *  specifies one (e.g. codex skills → $HOME/.agents). Undefined means the
+   *  kind installs under the runtime's normal configDir. */
+  home?: string;
 }
 
 interface Layout {
@@ -216,7 +220,7 @@ function createRuntimeArtifactInstallPlan(args: CreateRuntimeArtifactInstallPlan
     items.push({
       kind: kind.kind,
       sourceDir,
-      destDir: assertDestWithinConfigHome(layout.configDir, kind.destSubpath),
+      destDir: assertDestWithinConfigHome(kind.home ?? layout.configDir, kind.destSubpath),
     });
   }
 
@@ -227,7 +231,7 @@ function createRuntimeArtifactUninstallPlan(layout: Layout): UninstallPlan {
   return {
     items: layout.kinds.map((kind) => ({
       kind: kind.kind,
-      destDir: assertDestWithinConfigHome(layout.configDir, kind.destSubpath),
+      destDir: assertDestWithinConfigHome(kind.home ?? layout.configDir, kind.destSubpath),
     })),
   };
 }