feat: add Codex plugin support with hooks, commands, and documentation

.codex-plugin/skills/help/SKILL.md

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+---
+name: help
+description: Show MemPalace help — available commands, usage tips, and getting started guidance.
+allowed-tools: Bash, Read
+---
+
+# MemPalace Help
+
+Run the following command and follow the returned instructions step by step:
+
+```bash
+mempalace instructions help
+```

.codex-plugin/skills/init/SKILL.md

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+---
+name: init
+description: Initialize a new MemPalace — guided setup for your AI memory palace with ChromaDB backend.
+allowed-tools: Bash, Read, Write, Edit
+---
+
+# MemPalace Init
+
+Run the following command and follow the returned instructions step by step:
+
+```bash
+mempalace instructions init
+```

.codex-plugin/skills/mine/SKILL.md

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+---
+name: mine
+description: Mine a project or conversation into your MemPalace — extract and store memories for later retrieval.
+allowed-tools: Bash, Read, Glob, Grep
+---
+
+# MemPalace Mine
+
+Run the following command and follow the returned instructions step by step:
+
+```bash
+mempalace instructions mine
+```

.codex-plugin/skills/search/SKILL.md

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+---
+name: search
+description: Search your MemPalace — semantic search across all mined memories, projects, and conversations.
+allowed-tools: Bash, Read
+---
+
+# MemPalace Search
+
+Run the following command and follow the returned instructions step by step:
+
+```bash
+mempalace instructions search
+```

.codex-plugin/skills/status/SKILL.md

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+---
+name: status
+description: Show MemPalace status — room counts, storage usage, and palace health.
+allowed-tools: Bash, Read
+---
+
+# MemPalace Status
+
+Run the following command and follow the returned instructions step by step:
+
+```bash
+mempalace instructions status
+```