feat(tooling): run blog from Claude Code, OpenCode & Codex on shared engine

Extract newsletter scripts to a neutral scripts/newsletter/ engine and add
side-by-side support for all three AI coding tools off one source of truth.

- Move 9 scripts + substack config from .claude/skills/**/scripts to
  scripts/newsletter/ (history preserved); fix PROJECT_ROOT depth and
  cross-require/config paths; repoint all SKILL.md invocations
- Add canonical AGENTS.md; reduce CLAUDE.md to an @AGENTS.md import
- Add opencode.json (permissions, no MCP); skills auto-discovered in place
- Add codex/prompts/*.md (6 prompts) + copy installers (install.sh/.ps1)
- Add docs/multi-tool-usage.md (setup, per-tool invocation, teardown) + README pointer

opencode.json

@@ -0,0 +1,27 @@
+{
+  "$schema": "https://opencode.ai/config.json",
+  "permission": {
+    "edit": "allow",
+    "webfetch": "allow",
+    "websearch": "allow",
+    "bash": {
+      "node *": "allow",
+      "git status*": "allow",
+      "git add*": "allow",
+      "git commit*": "allow",
+      "git diff*": "allow",
+      "git log*": "allow",
+      "git mv*": "allow",
+      "hugo*": "allow",
+      "cat *": "allow",
+      "ls *": "allow",
+      "cp *": "allow",
+      "mv *": "allow",
+      "mkdir *": "allow",
+      "find *": "allow",
+      "grep *": "allow",
+      "curl *": "allow",
+      "*": "ask"
+    }
+  }
+}