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>

capabilities/codex/capability.json

@@ -27,7 +27,8 @@
           "prefix": "gsd-",
           "nesting": "flat",
           "recursive": false,
-          "converter": "convertClaudeCommandToCodexSkill"
+          "converter": "convertClaudeCommandToCodexSkill",
+          "home": ".agents"
         }
       ],
       "local": [
@@ -37,7 +38,8 @@
           "prefix": "gsd-",
           "nesting": "flat",
           "recursive": false,
-          "converter": "convertClaudeCommandToCodexSkill"
+          "converter": "convertClaudeCommandToCodexSkill",
+          "home": ".agents"
         }
       ]
     },
@@ -49,7 +51,11 @@
     "installSurface": "codex-toml",
     "writesSharedSettings": false,
     "permissionWriter": null,
-    "extendedHookEvents": [],
+    "extendedHookEvents": [
+      "SubagentStop",
+      "Stop",
+      "PreCompact"
+    ],
     "hostIntegration": {
       "embeddingMode": "declarative",
       "commandSurface": "slash-file",
@@ -66,6 +72,13 @@
       "stateIO": "filesystem",
       "transport": "mcp",
       "runtime": "node"
+    },
+    "hostBehaviors": {
+      "reapplyCommand": "$gsd-update --reapply",
+      "tomlConfigInstall": true,
+      "cleanupSkillSidecars": true,
+      "agentTomlFiles": true,
+      "frontmatterDialect": "codex"
     }
   }
 }