fix(ask-codex): restore codex-binary prerequisite, fix dead schema URL

The rebase that squashed into #33 dropped 'Requires the codex binary on
PATH' from plugin.json and marketplace.json while README.md and
skills/ask-codex/SKILL.md kept it, breaking the four-surface consistency
issue #10 requires. Restore the identical sentence to both, keeping the
plugin.json/marketplace.json descriptions byte-for-byte equal (required
by validate-plugin-ports.mjs's marketplace description check).

Also replace marketplace.json's dead $schema
(https://anthropic.com/claude-code/marketplace.schema.json, 404) with
the live SchemaStore Claude Code marketplace schema. Editor-only, not a
runtime blocker, but free to fix alongside the rest of this scope.

Author: AojdevStudio

.claude-plugin/marketplace.json

@@ -1,5 +1,5 @@
 {
-  "$schema": "https://anthropic.com/claude-code/marketplace.schema.json",
+  "$schema": "https://json.schemastore.org/claude-code-marketplace.json",
   "name": "agentic-utilities",
   "description": "Curated Claude Code plugins from agentic-utilities — a harness-agnostic collection of skills, plugins, and extensions across pi, Claude Code, and Codex. This marketplace exposes the Claude Code plugins.",
   "owner": {
@@ -54,7 +54,7 @@
     },
     {
       "name": "ask-codex",
-      "description": "OpenAI Codex CLI integration for Claude Code. Wraps `codex exec` and `codex resume` with model selection (gpt-5.4 / gpt-5.3-codex / gpt-5.4-nano), reasoning effort control (high/medium/low), sandbox modes (read-only / workspace-write / danger-full-access), session resumption, and stderr suppression for thinking tokens.",
+      "description": "OpenAI Codex CLI integration for Claude Code. Wraps `codex exec` and `codex resume` with model selection (gpt-5.4 / gpt-5.3-codex / gpt-5.4-nano), reasoning effort control (high/medium/low), sandbox modes (read-only / workspace-write / danger-full-access), session resumption, and stderr suppression for thinking tokens. Requires the `codex` binary on PATH.",
       "version": "0.1.0",
       "author": {
         "name": "Ossie Irondi"

claude-code/plugins/ask-codex/.claude-plugin/plugin.json

@@ -1,7 +1,7 @@
 {
   "name": "ask-codex",
   "version": "0.1.0",
-  "description": "OpenAI Codex CLI integration for Claude Code. Wraps `codex exec` and `codex resume` with model selection (gpt-5.4 / gpt-5.3-codex / gpt-5.4-nano), reasoning effort control (high/medium/low), sandbox modes (read-only / workspace-write / danger-full-access), session resumption, and stderr suppression for thinking tokens.",
+  "description": "OpenAI Codex CLI integration for Claude Code. Wraps `codex exec` and `codex resume` with model selection (gpt-5.4 / gpt-5.3-codex / gpt-5.4-nano), reasoning effort control (high/medium/low), sandbox modes (read-only / workspace-write / danger-full-access), session resumption, and stderr suppression for thinking tokens. Requires the `codex` binary on PATH.",
   "author": {
     "name": "Ossie Irondi"
   },