refactor(codex): drop the .codex/agents sidecar; rely on nested skill agents (#173)

Codex auto-discovers subagents bundled inside an installed skill's own
agents/ folder, so the separate .codex/agents/*.toml sidecar was redundant.

- cli: remove installCodexAgents/isCodexLikely and their install/update calls
- context.mjs: remove the CODEX_AGENT_MISSING self-heal directive
- build: drop codex agentFormat so no top-level .codex/agents is emitted; the
  nested in-skill .toml bundling is the whole delivery
- remove the tracked .codex/agents/*.toml and the gitignore exception
- docs + build.test.js updated for the nested layout
- CLI patch version bump; skill version unchanged

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

scripts/lib/transformers/providers.js

@@ -40,7 +40,9 @@ export const PROVIDERS = {
     frontmatterFields: [],
     includeVersion: false,
     writeOpenAIMetadata: true,
-    agentFormat: 'codex-toml',
+    // No agentFormat: the Codex subagent ships nested inside the skill's own
+    // agents/ folder (see CODEX_SKILL_PROVIDERS in factory.js), which Codex
+    // auto-discovers on install. No top-level .codex/agents/ sidecar is emitted.
   },
   agents: {
     provider: 'agents',