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.

.devin-plugin/plugin.json

@@ -0,0 +1,22 @@
+{
+  "name": "superpowers",
+  "version": "6.2.0",
+  "description": "An agentic skills framework & software development methodology that works: planning, TDD, debugging, and collaboration workflows.",
+  "author": {
+    "name": "Jesse Vincent",
+    "email": "jesse@fsck.com"
+  },
+  "homepage": "https://github.com/obra/superpowers",
+  "repository": "https://github.com/obra/superpowers",
+  "license": "MIT",
+  "keywords": [
+    "brainstorming",
+    "subagent-driven-development",
+    "skills",
+    "planning",
+    "tdd",
+    "debugging",
+    "code-review",
+    "workflow"
+  ]
+}