feat: add hono diff

[yusukebe]

hono diff shows what your uncommitted changes do to the app. It runs hono snapshot at the last commit and on the working tree, each in a temporary git worktree with fresh local data, and lists the routes that answer differently plus the routes added or removed. With snapshot, an agent has to remember to capture before the change; with diff it can ask at the end.

The worktrees share node_modules (linked) and get .dev.vars copied. The user's index, stash, and branches are not touched: the working tree is committed with a temporary index. The two snapshots run one after the other, since two Vite servers rebuilding the shared dependency cache at once stalled for minutes. On the blog app from the trials it takes about 3.5s and reports GET /admin 200 -> 303 and the new login routes.

The author should do the following, if applicable

• Add tests
• Run tests
• pnpm run format:fix && pnpm run lint:fix to format the code
• Add TSDoc/JSDoc to document the code