feat: add Devin CLI support

Devin CLI's `devin plugins install obra/superpowers` fails today because the
repo has no `.devin-plugin/plugin.json` manifest. Add the manifest (skills are
auto-discovered from the co-located skills/ directory), a Devin tool mapping
linked from using-superpowers' Platform Adaptation section, a README install
section, version tracking in .version-bump.json, a Codex-sync exclude for the
new dotdir, and a CI-safe test mirroring the kimi/antigravity test style.

Bootstrap rides Devin's native skill surfacing: every installed skill's
name + description is injected into the system prompt at session start with a
standing instruction to invoke matching skills via the native skill tool.
Acceptance test ("Let's make a react todo list") passes in a clean session:
using-superpowers and brainstorming auto-trigger before any code is written.

scripts/sync-to-codex-plugin.sh

@@ -48,6 +48,7 @@ EXCLUDES=(
   "/.claude-plugin/"
   "/.codex/"
   "/.cursor-plugin/"
+  "/.devin-plugin/"
   "/.git/"
   "/.gitattributes"
   "/.github/"