feat: template-based dual-target generator for Claude + Codex

Replace generate-codex.sh with unified generate.sh that produces both
claude/ and codex/ output from template source files.

Agent bodies use ${PLANS_DIR}, ${WEB_SEARCH}, ${SEARCH_TOOLS} placeholders
expanded per-target via envsubst. Skills and rules made tool-agnostic
(no Claude tool names or .claude/ paths). Orchestrate skill stays
Claude-only.

install.sh now symlinks from claude/agents/ instead of agents/ directly.
flake.nix adds gettext (envsubst) to devShell.

.gitignore

@@ -9,5 +9,6 @@ settings.local.json
 .DS_Store
 Thumbs.db
 
-# Generated Codex CLI config (derived from Claude source files via generate-codex.sh)
+# Generated output (derived from source templates via generate.sh)
+claude/
 codex/